FIPS stands for Federal Information Processing Standards. The U.S. Census Bureau uses the FIPS code to uniquely identify states (two-digit), counties (three-digit) and census tracts (six-digit plus decimal points). For example, the FIPS code for Jefferson County in Colorado is 08059. The "08" is the Colorado state code, and the "059" is the county code assigned to Jefferson County.
Many of MapViewer's U.S. boundary files use the FIPS code as the primary ID, which is included in many of the data files as well. The FIPS codes in the included MapViewer data files are the five-digit variety, so they go down to the county level.