It just means you’re using ‘simplex’ collision, and the shape of the collision isn’t the same as the shape of the mesh.
The easiest way to fix it is to open the mesh in the static mesh editor, and under collision select ‘use complex as simple’. Not advised it you’re peppering these all over the landscape.
The long term solution is to remake the collision, either in the static mesh editor, of some like Blender 