In one sentence: A 301 redirect is a permanent instruction from a web server that sends anyone who requests an old web address to a new one, and tells search engines the old address has been replaced for good.
Rename a page, restructure a site, move to a new domain, or switch from http to https, and the old addresses do not politely disappear. They are printed on a vehicle wrap, saved in a bookmark, sitting in a directory listing, and linked from somebody else's post. You cannot go around and collect them.
A 301 handles that for you. It is a rule on your server that says: when anyone asks for this old address, send them to this new one, and treat the move as final. The visitor usually never notices, because the browser follows the rule and shows the new page.
Say your old site had a page at /services/water-heater-repair.html and the rebuilt site uses /water-heater-repair. A 301 on the old address means the link on your Google Business Profile, the mention in a neighborhood forum, and the bookmark on a repeat customer's phone all still land on the right page instead of a wall.
Search engines treat the new address as the replacement and carry over the standing the old address had earned. That handoff is the real reason to set a 301 rather than let an old address quietly die.
Two practical notes. Browsers remember permanent redirects, sometimes for a long while, so a 301 set by mistake is stubborn to undo. And a redirect should point at the closest matching page, never the homepage by default, because the homepage does not answer the question the visitor asked.
Most weeks you will never think about redirects. The exception is the week your site changes shape: a rebuild, a rebrand, two sites merging, a move off an old platform. That is when addresses change in bulk, and it is the one moment where a missing rule becomes dozens of dead links at once.
The work itself is a map. Every old address, and the page that now answers for it. Old addresses with no real match get pointed at the nearest useful page or left to return a proper not found. A store makes the scale obvious. On a storefront with over 1,400 products and live Stripe checkout, built for a Virginia retailer, every product sits at its own address, which is 1,400 addresses that have to keep working whenever the structure underneath them changes.
Keep the chain short while you are at it. If an old address points to a second address that points to a third, every visitor and every crawler takes the long way around for no reason. Point the old address straight at the final one.
Redirect rules belong in the build itself rather than bolted on after launch, which is the plain reason they sit inside web development.
A 302 redirect is the temporary version of the same idea, and the difference is not cosmetic: a 302 asks search engines to keep the original address as the one they list. A 404 page is what a visitor gets when no rule exists at all and the old address simply has nothing behind it.
A canonical URL is a related but softer tool. It tells search engines which address you prefer when several show the same content, while a 301 actually moves the traffic. One is a preference, the other is a rule.
Keep it. There is no expiry date on an old link, and a redirect rule costs nothing to leave running. If an address is still being requested years later, the rule is still doing its job. The only reason to remove one is that you are reusing the old address for something else.
A 301 is built to hand the old address's standing to the new one, and it is the right tool for that job. What it cannot do is turn a thin replacement page into the page that earned the standing in the first place. The destination still has to answer the question the old page answered.
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