There are two types of heat maps you can build with Mapline: Radial Heat Maps and Territory Heat Maps. A Radial heat map looks like a weather radar where the center is a deeper color (red) and the outside is a lighter color (blue) so you can see the relation of your pins to each other.
The second type of heat mapping is Territory heat mapping. This is when you overlay a territory boundary on your map such as counties. Then you color each county according to the total locations inside that territory so that counties with a lot of locations are red and a county with few locations is green.