I don't see (what/why/how)(clause)

This either means that you don't understand something or that you don't agree with it. For example, if you're waiting for a long time on an airplane for it to take off, you can say to the person who's riding with you:

I don't see what's taking so long.

This means that you don't understand why it's taking such a long time, and you're angry about it. So "I don't see ___" is used when you don't understand the reason for something and you don't think that there really is a good reason.

Here are some more examples with different endings:

I don't see why not.

I don't see how that's possible.

a big deal

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.

Big deal!

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.