A safety pilot is a certified pilot who scans for other aircraft while another pilot practices instrument approaches or other maneuvers under simulated low visibility conditions while the practicing pilot wears a view limiting device that prevents the pilotfrom seeing outside the cockpit window. Under this arrangement both pilots might qualify for pilot-in-command (PIC) time and the practice approaches count toward IFR currency.
To be a safety pilot, you must have your private pilot certificate and be current (both medically and in the aircraft, i.e. takeoffs and landings in the last 90 days, etc) and checked out in the aircraft in which you are acting as the safety pilot. Additionally, the two pilots (the practicing pilot and safety pilot) should work out, before hand, how you are going to handle making radio calls and exchanging control of the aircraft.