Rotating an edge clockwise (CW) or counterclockwise (CCW) spins an edge between two faces around their vertices. This is very useful for restructuring a mesh's topology.
The tool operates on selected edges or the shared edge between selected faces.
To rotate an edge based on faces you must select adjacent face pairs, otherwise Blender notifies you with an error message, "Could not find any select edges that can be rotated". Using either Rotate Edge CW or Rotate Edge CCW will produce exactly the same results as if you had selected the common edge.