Tilly:
Use your mouse to move the blue bar, at the foot of the screen, left and right. The idea is to stop the ball reaching the bottom of the screen so that it keeps knocking out blocks above.
Dave has correctly stated that you can use the arrow keys instead of your mouse. Indeed, if you're trying to emulate the way that the game was played on machines (looking like coffee tables) in pubs, that the proper way to do it.
Huh! These young pups probably think they invented nostalgia.
I remember tellys where you had to push a button to change the channel.
Come to think of it, I remember the tellys with slot meters.
Ah, happy days.