A caterer was someone providing the service of catering, i.e. provide food for events and other gatherings of people. Quark occasionally provided catering for events on Deep Space 9, often personally.
In 2371, Quark offered to loan his new replicator if a caterer should be required on the dangerous trip the USS Defiant was about to make to the Gamma Quadrant to locate the Founders. (DS9: "The Search, Part I")
For his wedding with Kassidy Yates in 2375, Benjamin Sisko asked her permission to have Quark do the catering. She agreed as long as he didn't overcharge. Due to the ultimate circumstances of the wedding, Quark only had 20 minutes to notice. (DS9: "'Til Death Do Us Part ")