User info is a user script by PleaseStand that adds an information line below a user's name on their user and talk pages (but not subpages). Namely, this information is gender, privileges, account age, edit count, and how long ago the user made his/her last edit.
Two examples of what you might see:
User:Jimbo Wales♂A checkuser, founder, oversighter, and administrator, 9 years 1 month old, with 5277 edits. Last edited 7 days ago. From Wikipedia, the free encyclopedia
User talk:ExampleA blocked user, 20 minutes old, with 5 edits. Last edited 1 minute ago. From Wikipedia, the free encyclopedia