These buttons allow you to enter values into the quantity field. They are active when the POS is in manual mode and the item selected has not triggered the activation of the touch keyboard.
To increment (or decrement) the quantity click the +1 or -1 button. The other buttons add the character you click to the quantity already there. As in, if 1 is there and you click 2 the quantity field displays '"12", not "3". In other words the quantity buttons act like the buttons on a calculator.