When you build your first website, you will eventually encounter situations where something goes wrong: a page is not found, a form submission fails, or a server cannot respond. Most default error pages show a code like 404 or 500 and nothing else. A visitor who sees only a number has no idea what happened or what to do next, and many will simply leave. Replacing those defaults with messages that explain the situation in plain language is one of the simplest improvements you can make early on.
A useful error message answers three questions for the visitor: what happened, why it might have happened, and what they can try next. For example, instead of showing "404 Not Found" alone, you might write that the page they were looking for could not be found, that the address may have changed or been mistyped, and that they can return to the homepage or use a search feature if one exists. You do not need to explain the technical cause in detail; you need to give the person a next step.
Tone matters more than most new developers expect. Error messages are one of the few places where a frustrated or confused person interacts with your site. Writing in a calm, direct voice without blame helps. Avoid phrases like "illegal operation" or "invalid input" that sound accusatory. Instead, describe what the system needs: "Please enter a valid email address" is clearer and less frustrating than "Error: invalid input." The goal is to help the person correct the problem, not to make them feel at fault.
A practical way to review your error messages is to deliberately trigger each one yourself. Type a wrong URL, submit a form with missing fields, or disconnect your database and reload a page that depends on it. Read what appears on screen as if you were a first-time visitor with no technical background. If you cannot tell what to do next from the message alone, rewrite it until you can. Going through this process before launch can help you notice messages that still rely on jargon or dead ends, though the number of scenarios worth testing depends on how many distinct error states your site actually has.