Plain English Meaning
A service that takes what radar already sees on the ground and beams it up to your aircraft, so you can see other planes around you on a screen in the cockpit.
Full Definition
A ground-based service that uses Mode S radar data to send nearby traffic information up to a properly equipped aircraft, displaying surrounding traffic on a cockpit screen along with relative altitude, direction of travel, and approximate position. TIS is provided only within the coverage of certain terminal Mode S radar sites and requires a Mode S transponder with TIS capability and a compatible display.