A beer tap is a means of tapping a keg or a barrel.
This term can be a little bit broad. When someone is referring to a tap they may be referring to a valve, spigot, faucet, tap handle, or some other part of the tapping mechanism.
Most commonly when someone is referring to a tap in a bar they’re referring to the faucet or the whole tap system. When you’re in a situation where you need to tap a keg and you’re not at a bar the tap will normally be referring to a portable keg tap system.