A jungle is a forest thick with trees, other plants, and animals. Jungles are a little dangerous, which is what people mean when they say, "It's a jungle out there!"
Jungles — thick tropical forests — are full of life: birds, insects, reptiles, monkeys, and often gorillas and other animals. They're dangerous places, even for the animals that live there. This is why jungle also means any place that is risky or wild. If someone does something a dishonest or shady to get their way, some people will say that's "The law of the jungle," meaning you have to do some shifty things to survive.
