HTTP codes
Moved permanently
The address changed for good, and browsers are sent to the new one.
Also called: moved permanently, redirect, permanent redirect
301 is the status code for a page that moved permanently. The browser follows it to the new address without asking anyone, so the visitor sees the new page and nothing else.
Where you meet it
Search engines carry the old address ranking over to the new one. Skip these during a redesign and the traffic goes with them, which is the single most common way a new site loses visitors it already had.