Amphibians are animals that are like reptiles and fish. When they are young, they need to be in water like fish, or they will die. They don't die because they breathe oxygen from the water with their gills. Then, when they are adults, they have to breathe oxygen from the air to stay alive. They die if they try to breathe water as adults.
'Amphibian' comes from two Greek words: "amphi" and "bio". Amphi means "both". Bio means "life".
One example of an amphibian is a frog. A frog starts as a tadpole, which looks like a fish, with a tail. Then it goes through a stage called metamorphosis, and grows up into a frog, and it lives like a mammal or a bird, where it breathes air and only goes into the water sometimes.