You know that frogs come from tadpoles, right? That's the time it spends in water. They hatch from eggs in the water and grow into larvae with gills. As tadpoles, an amphibian can be an herbivore, meaning they only eat plants, or algae in this case. They only have one circulatory system for blood as well.
As juveniles, they begin to grow lungs and limbs. They develop a new digestive system, so they can eat protein, like bugs. Their circulatory systems also change. They grow a second loop for their blood, and a better heart.
Once they're adults, they spend their life on land, either hopping or crawling around on the ground. They now have legs, and arms. Also, they have lungs, now, so they can breath. Not only that, but they can blink, just like us!
So if you've wondered what it meant, now ya know!