I have never heard this phrase being used other than in a restaurant or fish shop context. Outside these scenarios, I would assume it's being used as a greeting in a similar way to "What's happening?" or "What's up (Whazzup)?". In other words, "What are you doing today?"
Depending on your personal circumstances and preferences, you might reply anything from "I'm off to Monte Carlo in my private jet", through "I'm hanging with my homies", to "Nothing much; what about you?"
Student or Learner