To jump is to hop or leap, straight up or over something. Frogs, amazingly, are able to jump up to twenty times the length of their body.
You can jump over a puddle or over the jump rope your friends are turning. You might jump in surprise or fear, like when a scary noise in the night makes you jump, or startle. You can jump off of something, like a diving board, or figuratively jump into an activity. If a bully jumps you, he attacks you suddenly, and if your brother says, "Go jump in a lake," he means "Go away, you're bothering me."