From SKYbrary Wiki
A Safety Pilot is usually used when a pilot new to an aircraft type commences line/route training on a multi crew aircraft type. Their role is to observe the overall operation of the aircraft from the supernumerary crew seat and support the Training Captain who is in command. They are also available if the Training Captain determines that the trainee’s performance does not meet minimum standards for an operating crew member. The Training Captain decides when such support is no longer required, subject to any operator guidance provided on the subject.