Plain English Meaning
If you tell Flight Service when and where you're flying VFR, and you don't check in within 30 minutes after you were supposed to land, they start looking for you. It's free, but only works if you remember to file the plan, open it when you depart, and close it when you arrive.
Full Definition
A free service provided by the FAA, in cooperation with the U.S. Air Force Rescue Coordination Center and the U.S. Coast Guard, that initiates a search for an overdue VFR aircraft when the pilot has filed a VFR flight plan and fails to close it within 30 minutes after the estimated time of arrival. The system relies on the pilot opening (activating) and closing the with Flight Service.