Bail. The bail on a spinning reel acts as a gate for the line on the reel spool. When the bail is engaged, fishing line is prevented from unwinding from the spool. When the bail is open, as it would be when casting, it releases line from the spool.
What is the knob on top of a spinning reel?
Starting at the top of the spinning reel, the drag system is what adds or removes tension to your fishing line. A knob on top of the spool (and sometimes, in the back) allows you to adjust the frictionless plates, which increases or decreases your tension. Having this adjustability can help when fighting fish.