Why does the ABS & Emergency brake light stay on all the time?

Why does the ABS & Emergency brake light stay on all the time? My truck only has 25,000 miles on a 2008 Ford F150 1/2 ton. Lately the ABS & Emergency light comes on 5 minutes after I drive off and stays on as I drive. The emergency brake is not on, except when I park, then release when I drive off.