When you say that something is "no big deal" it means that it's not a problem. It's not important for you to worry about it.
A: I'm so, so sorry.
B: Don't worry about it. It's no big deal.
You can also use "no big deal" to say that something isn't important. For example, if someone asks you about an award you've won, you can humbly say:
Yeah, it's no big deal.