Whats convexity and concave in art form?

That depends on if you mean 2D or 3D art. Convexity and Concave in 2D is about perspective, making the objects in the work looking like they are going away, or toward the viewer. In 3D art it is probably referring to the 3 layered freezes, in which the sculptor is using physical closeness, (parts of the freeze that is sticking out from the base of the freeze) and far (parts of the freeze that is carved out of the base of the freeze) to take one step further what the 2D artists try to do with perspective alone.