“What's the deal with this soup?”
You come home and your wife is watching TV. You see that there's a pot of cold soup on the stove, but you don't know whether it's good to eat or whether it needs to be thrown out. You ask this to find out.
What's the deal with this soup?
This is a question that you can ask to get general information about something. "What's the deal with ___?" isn't a specific question, so the person who you ask can give whatever answer is appropriate. In the situation above, the wife can give any of these answers:
Oops, I made that for lunch but I forgot to put it away.
It's an Italian recipe I came across the other day.
It's still good. Just heat it up a bit.
Ask "what's the deal with ___" for things that are a little strange or unexpected, when you want to find out information about them but you don't know what exactly to ask. You can also ask about people this way if they're acting strange:
What's her deal?
The word "deal" in this phrase is not used as a verb. It's a noun which means "situation" or "story".