In various arcade shoot em up games (such as Gradius), when you beat the
game, it allows you to start over at the beginning of the
game after viewing the credits. You keep the score you had when you beat the
game, but the enemies in the levels become harder and the
game becomes more difficult. This is called a loop. The game starts on loop 1 and when you beat the game it goes to loop 2 with higher difficulty and when you beat it again you start over with an even higher difficuly setting at loop 3 and so on.