A person that performs ground operations for an airline on the apron/tarmac or "ramp" as it is known in the industry. Rampers handle the loading and unloading of baggage and cargo, plus service the aircraft with power, water, heat/AC, and the lavatory.
Within an airline, the term generally refers to the whole of the fleet service employees, though within that work group itself it refers only to those agents that actually service aircraft at the gates. As such, some employees -such as bag runners and bag room agents- dislike the term as they don't consider themselves "rampers".
The gate agents were trying to get everyone boarded before the rampers were finished loading the plane.