"Concave" 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 example below shows a straight cedar fence after it has been left to weather for 4 years, with gothic post caps.
|Bottom of Fence|
|Heights||48", 60", or 72"|
|More||Other Wood Fence Styles|