Tap on the glass

Can mean two things. A hearty clink of two bottles being knocked together orrrrr... when your x boy friend comes to your work... preferably a school and starts tapping on the glass making the children feel like they are about to be shot dead (this is in America by the way) and the teacher as though she is about to be obstructed.

Playing games - an app - on a tablet computer or smartphone

Variations:
Tap tap: predominantly typing
Rub glass: not typing

To tap on the window of a car when you see the police. Done to give you the luck to not get caught. Originating in Parker, Colorado.