A reference to players who do the least glamorous things on the basketball floor. Players who do the "dirty work" tend to relish their role, and are very good at it.
Players who do the "dirty work" for their teams are guys who are typically not stars, but are willing and able to help the team by doing things that many players frankly don't want to do. Most players love to shoot the ball and score points, and many players, especially point guards, enjoy piling up assists.
There are certain things, however, that most players are not very good at, or are not willing to do. Some of those "dirty work" functions may include rebounding, setting picks, deflecting passes or fouling opposing players in a legal but forceful manner.
Dirty work is also sometimes referred to as "the little things". Players who do "the little things" that aren't flashy and don't always show up in the statistics, but help a team win are admired for this characteristic. Every good team needs one or more players who do the dirty work in order to be successful.