LET THE CAT OUT OF THE BAG
Meaning: to give away a secret
What do a cat and a bag have to do with giving away a secret? Picture this. It's England, centuries ago. You go to a farmer's market to buy a little pig. But what if the pig seller cheats you? When you aren't looking, he puts a stray cat into a bag instead of an expensive pig. He hands you a sealed bag and tells you not to open it until you get home. Otherwise, the pig will run away. At home, when you open the bag, a cat jumps out instead of the pig you paid for. When you let the cat out of the bag, you revealed the secret of the dishonest merchant.
An idiom is a special kind of saying. The tricky thing about idioms is that the words in the saying don't always tell you what the saying actually means. Usually there's an interesting story that explains that. Here's an example of a famous idiom.