A Character Slot is a slot allowing you to create a player character.
Each user account inititally has 2 character slots, which can be used for creating Federation faction characters. A 3rd slot is unlocked when one of your characters reaches Lieutenant 6 as well as the ability to create Klingon faction characters.
Lifetime subscriptions bought before release included 2 extra slots.
Additional slots will be available for purchase through the C-Store.