Those are the arguements passed to the function when it is triggered, if that's a good definition.
From your example, when a mouse button is pressed, the values in parenthesis are passed to the function for you to use. I.e. 'button' is the value corresponding to which button has been pressed, left or right. I'm not totally sure what 'shift' is, but 'x' and 'y' are the coordinates of the point the cursor was at when the button was pressed.
The 'As Integer' part means that the arguements are being passed to the function as integer-type variables.
Someone please correct me if any of this is wrong!