If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
The purpose is to defend the apple from the "invaders" - bugs and other nasty insects that crave for your apple. To do this, click at the insect with your mouse and be in hurry as they attack very quickly. The game is online and absolutely free. Have a lot of fun!
Click on the pest trying to eat the apple.