Complex sentence: There is also a gold frog hidden in one level in each zone; for each gold frog you find, you unlock a new zone, and finding every gold frog in the game will unlock an alternate ending sequence.
Split into simpler sentences: There is also a gold frog hidden in one level in each zone; the player will unlock a new zone for each gold frog that is found. Finding every gold frog in the game will unlock an alternate ending sequence.