An idiom is a common expression that can’t be taken literally. But it makes the meaning understood.

*Good luck – Break a leg
Here, the speaker is not asking his friend to break his leg, but wishing him all the best.

*I am very happy- I am on cloud nine.
Here, the speaker is not on the cloud nine rather,he is showing his happiness for the event that has taken place in his life.

*It’s raining heavily – Its raining cats and dogs.
Here, the cats and dogs are not falling from the sky.It represents that its raining so fiercely that it resembles the fight between cats and dogs.

* I am really mad- My blood is boiling.
The speaker is in such a state of anger, that his outrage represents boiling of blood.

*I am listening to you- I am all ears
Here, the speaker is cautiously listening to the other person.

* I am going to the bed- I am going to hit the hay.

*To sleep like a log- To have a sound sleep.
Here the speaker wants to say log is a metaphor for sleep, as the log cannot be moved easily.Similarly, the person in deep slumber can’t be waken up easily.