Surface
A surface in UnrealEd is a partial plane in a 3D viewport, that can be selected with a single click. The term is sometimes confused with a Texture (UT), which is the image applied to a surface.
The polycount is the number of surfaces, either currently visible or in the map as a whole.
See Surface Properties window and Surface Flags (UT) for the settings that can be applied to surfaces in UnrealEd.