How to Explain Code Clearly (To Yourself or Anyone Else)

You open a file. It's fifty lines of nested conditionals, a couple of regexes, and a function name that tells you nothing. You know it does something important, because half the app breaks if you touch it. But you can't explain what it does in one sentence, and that's a problem, because if you can't explain it, you can't safely change it.

This happens constantly: reading someone else's pull request, inheriting a service nobody documented, or coming back to your own code after six months away and not recognizing it. The skill that gets you out of that spot isn't "read faster." It's explaining code well, to yourself first, then to whoever needs to understand it after you.

Why Explaining Code Is Harder Than Reading It

Reading code and understanding code are different activities. You can read every line of a function, understand each statement in isolation, and still have no idea what the function is for. That gap is where bugs hide and where onboarding time gets wasted.

Explaining forces a level up. To explain something you have to compress it: strip out the incidental details (this variable is called tmp, this loop uses a for instead of .map()) and keep the essential ones (this function validates a payment amount before it hits the database). If you can't do that compression, you don't understand the code yet, you've just read it.

Start With What the Code Is For, Not What It Does

A common mistake is explaining code line by line, top to bottom, as if reading it in order. That mirrors how the computer executes it, but not how a person understands it. Instead, answer these first:

1. What problem does this solve? Not "it loops through an array," but "it dedupes duplicate orders before they're charged twice."
2. What are the inputs and outputs? What comes in, what goes out, what side effects happen (a database write, an API call, a file saved)?
3. What would break if this didn't exist?

Once you can answer those three questions, the line-by-line detail is much easier to follow, because you know what you're looking for.

Explaining Code at Different Skill Levels

The best explanation for a senior engineer and the best explanation for someone six months into their first job are not the same explanation. This is where a lot of documentation and code comments fail: they're written at one level, usually the author's level, and everyone else either gets too little context or wades through explanations of things they already know.

For a beginner, useful explanations name the concept explicitly:

// This is a "debounce": it delays running fn until
// the user stops calling it for `delay` milliseconds.
// Used here so the search API isn't hit on every keystroke.
function debounce(fn, delay) {
  let timer;
  return (...args) => {
    clearTimeout(timer);
    timer = setTimeout(() => fn(...args), delay);
  };
}

For someone experienced, that comment is noise. What they actually want to know is the non-obvious part: why 300ms and not 500ms, why this doesn't use lodash's version, whether it's safe to call from multiple components. Skill-appropriate explanation means adjusting what you assume as known and focusing your words on the part that's actually surprising.

This is exactly the gap that tools built for explaining code try to close. ExplainThisCode lets you set a skill level and a role (frontend, backend, data, and so on) so the explanation it generates matches what you actually need, instead of a generic paraphrase of the syntax.

A Simple Framework You Can Reuse

When you need to explain a function, class, or file, walk through these five questions in order:

• Purpose: What problem is this solving?
• Inputs: What does it need to run, and where does that come from?
• Process: What are the two or three key steps? Not every line, just the ones that matter.
• Output/side effects: What does it return, write, send, or change?
• Gotchas: What's the one thing that would surprise a new reader? A hidden mutation, an async call that's not awaited, an edge case that isn't handled?

Applied to a real function:

def apply_discount(cart, code):
    promo = get_promo(code)
    if not promo or promo.expired:
        return cart
    for item in cart.items:
        if promo.category is None or item.category == promo.category:
            item.price *= (1 - promo.percent_off)
    return cart

• Purpose: apply a promo code's discount to matching items in a cart.
• Inputs: a cart object and a promo code string.
• Process: look up the promo, bail out silently if it's missing or expired, then discount only items matching the promo's category (or everything, if the promo has no category restriction).
• Output: the same cart object, mutated in place, not a new one.
• Gotcha: an invalid code fails silently and returns the cart unchanged, so there's no way for the caller to know the discount didn't apply. That's worth flagging in a review, not just explaining.

Notice the gotcha is the most valuable part of the explanation. Anyone can read the loop. Spotting that silent failure is what separates a real explanation from a summary.

Explain in Writing, Not Just in Your Head

If you explain code silently in your head and move on, you've helped yourself for about a week. Write it down, in a comment, a commit message, or a short doc, and you've helped everyone who touches that code after you, including future you.

You don't need to over-document. A single sentence above a non-obvious function, or a short paragraph in a PR description explaining why (not what, the diff already shows what), is usually enough. The goal isn't exhaustive documentation, it's making sure the reasoning that lived in your head at the time doesn't disappear the moment you move on.

Practice on Code That Isn't Yours

The fastest way to get better at explaining code is to explain code you didn't write, regularly. Pick a function from an open source project or a legacy file at work, and write a three-sentence explanation: purpose, key steps, one gotcha. Do this a few times a week and the skill compounds fast, because you stop being able to hide behind "I wrote it, I remember what it does."

Key Takeaway

Explaining code well means compressing it down to purpose, inputs, process, output, and the one thing that would surprise a new reader, pitched at the level of the person who needs to understand it. That's a different skill from reading code, and it's the one that actually prevents bugs, speeds up onboarding, and makes your own code easier to come back to.