Yes, you didn't read it wrong! Lightmass is the name Unreal engine usually gives to the term lightmaps. When it comes to baking the light data into maps (the lightmaps/lightmass), Unreal Engine has within its rendering toolset: the ability to bake the light data into this type of maps. These maps will be used later at runtime to light the level. The lightmass builds the lightmaps for the stationary light sources and the static light sources only.
And it is obviously clear why it does not supporting the dynamic (moveable) lights, as this type of lights can't be precomputed by any means.