Big Deal
= something very important
Ex: This party is a “big deal.” I want to look my best.
Ex: This is the final test of the year, so it’s a “big deal.”
What’s the big deal?
= why is this bad / important ?
Ex: “What’s the big deal” with gay marriage? I don’t think it’s a problem.
It’s no big deal / it’s not a big deal
= it is not serious / important