When you tumble, you fall abruptly. You might trip over your shoelaces and tumble down a hill while flying a kite if you're not careful.
To suddenly topple over or trip and fall to the ground is to tumble. Toddlers often tumble in sandboxes, while running on sidewalks, or as they climb down stairs. There's another way to tumble that is more deliberate: the form of gymnastics that involves roundoffs and back handsprings is called tumbling, and if you're able to do these tricks, you can tumble. The root is believed to be the Old English tumbian, "to dance about."