"Convex" refers to the shape of the curve on top of the fence. If the the arc is higher in the center than at the posts it's convex. If the arc is lower in the center than at the posts it's concave.
The photo below shows an example with 3" board spacing, made of cedar lumber with amberwood stain.
|Wood Choices||Cedar or Pressure Treated|
|Bottom of Fence|
|Heights||3', 4', 5', 6', 7', or 8'||Board Spacing|
|More||Other Wood Fence Styles|