A situation is "a big deal" if it's something that people are treating as important. If people are getting very excited, very angry, very worried, or any other strong emotion about an issue, you say that issue is "a big deal".
Some common phrases that include "big deal":
What's the big deal?
This is something you ask when a person is getting upset over a topic, but you don't understand why.
Don't make a big deal out of it.
You say this when someone is getting very excited or upset about an issue and you want them to relax.
This is a very big deal.
You say this when you're talking about something that's very important but other people don't realize that yet.
This is a response that's similar to "So what?". You say this when you want to show that you don't care about what someone just said.