From Halopedia, the Halo wiki
Heatmaps are the Doppler Radar System of Death in Halo 3 and Halo: Reach. They track encounters, weapons used, and their results in a given game, collecting that data and sharing it with players visually. The key to heatmaps is, “the darker the red, the more frequent the deaths" (or kills, depending on the parameters).
You can find your own Heatmap statistics at Bungie.net. By visiting your Career Stats page, you’ll see a new tab labeled Heatmaps. Once you click that, you’ll be able to click each of Halo 3’s maps on the left side and see all manner of statistics, what weapons you’ve killed with and where you’ve used them on a given map, and where you’ve died to a certain weapon on a given map all via sortable dropdown menus. These heatmaps, however, aren’t a collection of your lifetime statistics.
Heatmaps can be used by players to check where both they and friends frequently die, and can thus rethink their strategy as to where they should position themselves when in matchmaking.
The dropdown box that has numbers in it is to get a general idea of where people are getting killed or are dying at, or to get a better picture. The higher the number, the broader the view.