if event is InputEventScreenTouch and event.pressed == true:
(event.position) # the position of the touch
InputEventScreenTouch and things like
InputEventKey and so on just describe the input event, so you use
if event is Input... to first check if the event is the right one, before running any code on it (or else you'll get errors).
In this case it's also necessary to check if
event.pressed == true or else the code will run also run when the player un-touches (twice in one tap).