Heatmaps are the Doppler Radar System of Death in Halo 3. they track encounters, weapons used and their results in a given game, collecting that data and sharing it with players visually. The key here 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 (and where you’ve used them) with on a given map, 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(dying) at, or to get a better picture. the higher the number, the broader the view.