Most websites that requires user login now utilize a "token" system. Every time a user logs into the website, they are issued a token so that when they go to the website again, they are authenticated with the token so they don't have to log in again. Usually a token has an expiration time so that after certain amount of inactivity, the token is voided.
This is a companion discussion topic for the original entry at https://algo.monster/problems/twitter_oa_auth_token/