Epi-pens are used to revive unconscious players. They can be found in military and deer stand spawns, but are more commonly found in hospitals. Epi-pens take up a single inventory slot, and are used by choosing the "Give Epi-Pen" option from the mouse wheel menu on an unconscious player. Epi-Pens cannot be used on the player by themselves.
This page contains information about the mod version of DayZ. Standalone information is available here.