No big deal means 'not important'.
'A big deal' is something important.
I'm only five minutes late - don't make a big deal out of it.
Ed's party is going to be a really big deal. Everyone's going.
I lost my book, but it's no big deal - I'll buy a new one.
You can also say 'big deal' in a sarcastic way to show that you think something isn't very good, important or interesting.
I got a 'b' in my test.
Big deal! Who cares?
Hope this helps,
- For Teachers