Like when i work on the game i start the cool down and when i stop i pause it ? Or does it means i should make a game within 48 hours since i have started (sorry for english)

You have 48 hours non-consecutive to work on the game. That's why the jam is about a week long, so we can have a chance to put in the whole 48 hours. Whenever you stop working, you pause the cooldown. When you resume your work, the clock keeps ticking!