From the manual
To get Frogger safely into the required number of home berths. Home berths are dispersed among the three different screens: underwater, on the water's surface, and in the air. In Level 1 for instance there are five home berths; three in the underwater screen and one each in the surface and air screens. Frogger need not reach any of the home berths in one screen before he goes to the next. As you'll see, Frogger may travel from one screen to another without loss of life. To advance to the next highest diffliculty level, however, Frogger must fill all open berths in each of the three screens.
There are bonus points to be had in each screen, too. But you'll collect them only if you take frogger to on open berth on that same screen!
In this game, Frogger has five lives. The number of remaining lives appears at the top of the screen. The remaining number of berths to be filled appears at the bottom of the screen. Froggers' got a time band to watch out for too. If he doesn't make it to a berth by the time the band runs down it's bye-bye Frogger. Before this happens, however, you'll hear a warning sound and see the band at the top of the screen turn red.
When Frogger loses a life. he returns to the first screen with his remaining lives.