When do you say that? How do you say it another way?
The cat's out of the bag.

Question

When do you say that?  How do you say it another way?
The cat's out of the bag.

Answer

Yes Taima is right! Imagine a bag and you don't know what's inside, and suddenly - a cat appears! 
You have told somebody some information - perhaps a secret - and now the truth is out - 'the cat has been let out of the bag' - the cat's out of the bag! :)

Answer

This idiomic phrase used when  a secret revealed  by someone.
We can also say: "Let cat out of the bag"(  means to reveal a secret).