It’s tracing where candidates predominate, with blue and deep blue being Sanders - the impression is given, by the presence of blue across a large swathe of the country, that Sanders is winning an overwhelming victory. However, much of the area that is blue is not considerably populated; while Sanders enjoyed plurality support at this point in the primary (I remember it well, because I was so fucking excited and hopeful), it’s not even vaguely close to the implication of overwhelming support that the map gives off.