To pounce is to attack suddenly by leaping onto your prey. A tiger will slowly sneak up on its prey and then suddenly pounce, using speed as much as strength to make the kill.
Of all the animals, cats are the best at pouncing. Snakes strike. Elephants charge. But when an animal leaps onto its prey, that's pouncing. When we talk about how people pounce, however, we're usually talking more figuratively. When we say that a person pounced on another person, we mean that they attacked viciously without warning — usually giving the victim no time to defend themselves. Lawyers pounce on witnesses, for example. If you feel suddenly ganged-up on, you might feel pounced upon.