"Walking Speed" is how fast or slow a unit moves when they walk on the map. Most units have a "normal" walking speed. Some units have a slow walking speed. These slow classes/units are:

Hector Great Lord*, Knight, General, Shaman, Druid, Transporter, Dark Druid, Civilian, and the Tactician class.

*(this class only has the class name "Great Lord", everything else is still unpromoted. It is not the actual Great Lord class.)