A tumble can be a fall or a tangle, but usually, when people say "I took a tumble" they mean they fell down.
Imagine playing outside on a bike. You might take a tumble if your wheels get stuck in mud and you fall off. Or maybe you're just clumsy and trip over something - that's a tumble too. Sometimes people talk about a tumble dryer, which is just another word for a clothes dryer that helps spin out the water from your clothes.
A fall, especially end over end.
Example: I took a tumble down the stairs and broke my tooth.
