This phrase is essential of you are travelling to or living in a West Indian neighbourhood .(that’s ‘neighorhood’ to all the Americans in the house). It means, ‘What’s going on?’ in the Marvin Gaye sense.
It has a few versions, namely Wa’ Gwaan, ‘Wa’pn’, and can be used in many situations. like the one illustrated below;
At the African Carribean market when buying your weekly supply of yams,
Shopkeep: Wha’ gwaan. You wan a yam?
You: Na, me no wan a wole yam.