Pachinko Balls are tiny silver balls that are useable in the Pachinko min-game. Playing pachinko gives you the opportunity to win coins, cards, powers, food, revives, and even gems! The game is accessed on the shop screen in the bottom right corner, and the game itself is quite simply as you launch pachinko balls by holding down or swiping the launcher. The more balls you have in play, the bigger your multiplier, so it's best to wait until you have a good amount to highten your chances of winning good items.
You can gain pachinko balls by buying a pack of them (not recommended), or gaining them during waves. They appear on the screen as silver balls and you must collect them. You will sometimes get huge rewards of these during bonus rounds and even during waves, so just collect them when you can and then use them in the game. Once you launch your 1000th ball, this achievement will unlock.