“Sorry I lost the book you lent me.”

“Don’t worry; it’s not a big deal.”

A big deal is something very important or meaningful. When we use this phrase, we usually say things are not a big deal. For example, when someone apologizes for a small mistake, we say it’s not a big deal, so they know the mistake doesn’t bother us. Or if we accomplish something but want to be modest, we say our accomplishment is not a big deal.