>What's the deal? -Co to znaczy?
>Można przetłumaczyć to w ten sposób "O co chodzi?" ?
What's up with "O co chodzi?"
What's the deal? = What's the understanding/bargain/agreement/...(in business or elsewhere.)
If you know this much, you will know what the slang deal can mean:
Deal: basic info about something or a person.
What's the deal? = What's the situation?
What's the deal with that/him? = What's the story/situation with that/him?"
What's the deal for tonight? = What the situation for tonight? What are we going to do tonight?