301 redirect
A 301 redirect permanently forwards one URL to another, sending visitors to the new address and telling search engines to transfer the old URL's signals. It is the mechanism that lets a site restructure or migrate without losing rankings.
Every URL that ranks or has earned links is an asset. When a page moves, the 301 is how the asset follows it: browsers land on the new address, and search engines consolidate the old URL's history onto it.
Migrations live or die on this. A redirect map, every old URL paired with its new destination, written before anything switches, is the difference between a boring migration and months of recovering lost traffic. Bulk-redirecting everything to the homepage is the classic shortcut, and search engines treat those as soft errors rather than transfers.
The details that bite: redirect chains (A to B to C) waste crawl budget and dilute the signal, 302s signal “temporary” when you mean permanent, and forgotten query-string URLs quietly 404. The map catches all three.