Tumbling in gymnastics is when someone does flips and jumps and other cool stuff on a mat or a bouncy thing called a trampoline.
To do a flip, you have to jump really high and then tuck your body in and roll over in the air. It's kind of like doing a somersault, but instead of rolling on the ground, you do it while you're still in the air.
Some people can also do twists, which is like doing a flip but you turn your body around while you're in the air.
Tumbling is really cool to watch, but it takes a lot of practice and skill to do it safely. That's why gymnasts train really hard and have coaches to help them learn how to do all the different moves without getting hurt.