Plain English Meaning
FSS stations are the FAA service desks pilots talk to for weather, flight plans, and general help before, during, and after a flight. They give information and assistance, but they do not direct traffic the way a tower or center does.
Full Definition
An FAA air traffic facility that provides pilots with a wide range of flight-related services, including weather briefings, flight plan filing and activation, in-flight assistance, search and rescue alerting, and the relay of clearances and advisories. FSS personnel do not provide air traffic control separation services; their role is information, planning, and assistance rather than directing traffic.