What is concave on a skateboard?
the concave is how much the board curves inward toward the middle starting at each side. so if you can imagine looking straight down the skateboard holding it level to your eyes with the nose in front of you, you would see the shallow dip the board has compared to being just flat.
it helps with flick, it's really more of a preference whether you like it shallow or deep or anything. Corey sheppard i once read runs his over with his car so they flatten out.
i guess if you did a lot of flip tricks though youd want it more curved to catch your flick more.