The site displays locations for 10 popular chains: McDonald's (with 13,642 locations shown), Burger King, Pizza Hut, Taco Bell, Wendy's, KFC, Jack in the Box, Hardee's, Carl's Jr., and In-N-Out Burger.
The site is navigated by selecting a state and then city in the upper-right corner of the screen. You can then navigate the map using the standard Google Maps controls (dragging the map and zooming in and out). The 'screen stats' at the lower right show the restaurant totals for the area shown on the map screen. Here's the map, locations and stats for Fort Worth, Texas (my home).
There's also a static US map that shows the density of all or selected restaurant chains across the entire US.