The asteroids represent NANO transactions of .01 or higher; brown asteroids represent send transactions, grey asteroids represent receive transactions. The number next to each asteroid is the transaction amount.
You gain a level every time 5 asteroids are destroyed. Each new level increases your scoring bonus, but also increases the speed of new asteroids.
On a PC, use the [←] and [→] keys to rotate your ship, [↑] key to thrust, and [space bar] key to shoot lasers.