Simply put, this symbol shows an obstacle at an elevation measured in MSL (Mean Sea Level). It is listed in MSL because the pilot is more concerned with his Altimeter reading than converting to feet above ground level (AGL), so MSL is given. The bigger dot inside a bigger wedge indicates the Highest obstacle on the diagram.

Dots with NO wedge indicate measured ground elevations at that particular spot given in Feet MSL.

Hi-Lighted is an obstacle 4,229 feet above Mean Sea Level.