What is meant by saying dont let the cat out of the bag?

It basically refers to secrets.

Cats, perhaps more so in popular perception than other animals, are naturally very uncomfortable when confined, say in a bag. The moment the cat has a chance, it's gonna get loose and try very hard not to get caught again. In other Words, once the cat is out, good luck trying to get it back in.

Such is with secrets. If the cat's out of the bag, the secret is out. Similar idioms relating to secrets refer to pigs ("Don't squeal."), beans ("Don't spill the beans."), and covers ("Keep a lid on it"), among other things.