301 redirect
A 301 redirect signals that a URL has permanently moved. It transfers the bulk of acquired signals — links, history — to the new address and prevents existing links from losing their traffic.
In practice
Essential in any site rebuild: every URL that changes without a 301 starts over from zero. Redirect chains (A to B to C) dilute the transfer and slow crawling; always redirect to the final destination. A 301 pointing to an unrelated page is treated as a soft error — the engine ignores the signal transfer.