As the tagline suggests, it shows your Google Location history visualized as a heatmap on an interactive map. E.g. My San Francisco location history: http://imgur.com/nN4frne, My now non-existent commute: http://imgur.com/WExNeBH
This seems like a great tool. I can imagine there are a lot of other services that could benefit from the heat map and just import the data with the same structure as good.
Shyp