Tadpoles are the stage of frogs' life cycle after they have hatched and while they still have gills and a tail.
A frog is an amphibian, and all amphibians lay eggs in water. The big clutch of eggs laid by a frog is called frog spawn, and it floats and feels like jelly. Inside the food sac, the egg develops into a tadpole.
When a tadpole hatches, it uses its tail to swim. It has a mouth on its underside, and it uses this to scrape tiny green plants called algae from the surfaces of rocks and plants. At this stage it has gills and breathes water. It is a vegetarian.
After a few weeks it develops hind legs and later on front legs. Then the tail withers and the gills change to lungs and the tadpole becomes a tiny frog. It is now a meat-eater (carnivore) and will eat insects and worms. A frog that has just stopped being a tadpole is smaller than your fingernail and hard to see in the grass by a pond, which perhaps explains why you may have wondered where all the tadpoles go to.
Eggs are laid in large numbers because the frog does not protect its young. So by laying eggs in large numbers, and by there being swarms of tadpoles born, it is likely that out of thousands, just a few will grow up to be breeding adults. All of the rest will be eaten because tadpoles are simply part of a food chain.