What does the idiomatic expression matter mean?

"Matter" in the expression "It doesn't matter" means "to be of importance or consequence". So, when someone says "it doesn't matter", they are saying that the situation or thing being discussed is not important or relevant.

Here are some examples:

* "What do you want to eat?" "It doesn't matter, I'm not that hungry." (The person is indifferent to what they eat.)

* "Should we go to the beach or the park?" "It doesn't matter to me." (The person is not bothered about either option.)

* "You forgot to bring your keys. It doesn't matter, I have a spare." (The forgotten keys are not a problem because there is a solution.)

The expression "it doesn't matter" is often used to express indifference, lack of preference, or a dismissal of a situation's importance.