Some bridge officers have "[Class] Team" powers: Tactical Team, Engineering Team, Science Team. These are self-or-ally-targeted powers (you can use them on yourself or any ally) that provide temporary boosts to certain things.
Tactical Team helps repel boarding parties and boosts damage (I think).
Engineering Team boosts hull repair rate and damage resistance.
Science Team boosts shield strength and something else.
By increasing your Team Leader skill in the appropriate area, you increase the effectiveness of these powers since adding skill points to your bridge officer for that skill only reduces the cooldown.