If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
The asteroid base is being attacked by enemy ships. You should deploy the magnetic shield to repel the attack. When you move the mouse with the button pressed a magnetic shield will be formed for a limited time. The fire will bounce in the shield and will revert against enemy ships. Sometimes you can get special powers-ups.
Use mouse to draw magnetic shield.