Q: What did the tiger say to her cubs before they went out to play?
A: Be careful–it’s a jungle out there!

Explanation: A jungle is a forest in tropical areas.  In a jungle there are many trees and plants that no one takes care of because they grow on their own.  There are many wild animals and birds in the jungle; there are lots of insects, too.  A jungle can be a dangerous place for someone who is not familiar with it.

In English, “it’s a jungle out there” is an idiom that means that a place or a situation is dangerous or threatening.

This joke is funny because the tiger in the joke is actually in a jungle.  No need to use that idiom in the jungle.

Check out this tiger cub in the jungle: