A ground handling agent does everything needed to turn an aircraft round, from it pulling up on stand to the towbar being disconnected after pushback. An agent normally meets the aircraft, coordinates ground services such as catering, fueling and toilet servicing, gets the pax/freight to and from it, provides documentation such as loadsheets, gen decs (customs) and pax name lists, and liaises with the crew throughout, covering specific requests and requirements where possible. Check in, freight acceptance and passenger welfare are also looked after by the handling agent.
As with anything though, some are better than others....
See, JB, I
was listening.