Service is frequently a component of dominant/submissive and master/slave relationships. It is where the submissive or slave assists or performs tasks or work for the benefit of the dominant or master. This may be done when explicitly commanded by the master or dominant, or may be done when the slave or submissive notices an opportunity such as to attentively pour wine or serve food during a meal.
As noted in the article elsewhere on this site about penetration, BDSM activities are invariably two-way. For example, a submissive may serve in some way and this should obviously benefit their dominant, but just as important is that this act of service should also be rewarding and satisfying for the submissive as well. Both people should feel the results of the service, particularly in a BDSM sense.
In service, the person performing the service, or servant, feels the commands or expressed desires of their partner. That is, their partner tells them what to do, or what they'd like done, and this determines the actions which the servant performs. In the same way that sexual intercourse requires more than one thrust (except in the case of teenagers), and a good flogging session requires more than one stroke, service requires more than one command.
Indeed, it is frequently necessary for there to be a more-or-less regular flow of commands, direction, or influence from the dominant or master for the service to continue to be rewarding for the submissive or slave. As a thought experiment, imagine what it would be like if a master gave his slave a single order and then never her saw her again. What it would it be like for her to continue to follow that order ad infinitum? Would it remain satisfying ad infinitum? Probably not. Therefore, service is an ongoing process of command and service. It is a process of continuous engagement.
Just as the master or dominant penetrates his slave or submissive with his orders and directions, his partner penetrates him with their service, and how they perform it. It is not commonly the case that a master or dominant will engage his partner in impersonal service, i.e., with service which doesn't impact him. Mostly he will engage his partner in service which he directly feels and from which he directly benefits.