A thin opening or groove in something. You might use a slot to put mail through at the post office, or to open a door.
In football, a slot receiver is an integral part of any offense. Without one, quarterbacks would have a hard time spreading the field and attacking all three levels of defense. Slot receivers are known for their route running and timing with the quarterback, but they also play an important role in the blocking game. They are usually lined up close to the middle of the field, so they will need to block (or at least chip) nickelbacks, outside linebackers and safeties on running plays that go to the outer part of the field.
Air Traffic Management
In aviation, a slot is a scheduled time for an aircraft to take off or land at an airport during congested periods. Airlines must bid for slots, which are allocated by an airline coordination agency (usually EUROCONTROL) to address runway capacity limitations or airspace congestion. Since their introduction in Europe in the 1990s, slots have led to substantial savings in terms of flight delays and fuel burn, and are now used worldwide.