"money 4 card" - some random kid
The Keycard is an item essential to Jailbreak, allowing you to access the bank to rob it, or enter the Police Headquarters/Police Station to steal guns or a helicopter. There is also a relatively unknown escape method in the prison's cafeteria, where you can crawl through vents that requires this item to access.
Pick-pocketed from police.
Use this to rob the bank as a criminal.
This can be used to escape the prison and getting guns from the Police Headquarters.
If you are a police, a notification will display that you had your keycard been stolen by a criminal/prisoner.
This can be used to farm cash by arresting prisoners after they stole your keycard.
Stash this at the storage bins in the cells to use later on and to prevent from getting arrested, but other players can take it if they know its there.