There are many threads on this where you can get even more info. Just search "swap meet".
My favorite part were items that were clearly Hawaiian and local, such as the custom wood carvings or some of the other artsy stuff. These are outnumbered by numerous tents with Asian-made incredibly inexpensive items such as keychains and common souvenirs. You can absolutely haggle if you like that sort of thing. They'll bring their own prices down if you just stand there silently and chew on your lip after you've asked about something.
Deals are supposedly better to be had on Wednesday than the weekends.
Not much to eat or drink. Don't come hungry and bring a water. There are coconut sellers- they'll crack one and give you a straw for $5.
Personally, I would not call it a "must-do". It's a decent diversion for 2 hours max.