The area in an offensive formation between the offensive line and the player closest to the sideline. This area is usually occupied by a wide receiver, running back, or tight end. The slot is often used to create formations that have multiple receivers on the same side of the field. Slot receivers are often smaller and quicker than the receivers who line up on the outside.
The slot is a common sight in modern-day passing formations. The slot receiver forces teams to adjust their defense to guard extra players, either by making the team bring in extra defensive backs, or by adjusting their current alignment to guard the slot. Slot receivers tend to run quick routes in the middle of the field, and they can create mismatches against linebackers or safeties who have to come up to guard the extra receiver.
The other advantage to the slot is that having multiple receivers on the same side of the field can create confusion in the defense, and plays can be designed to put the defending players in difficult situations.