The 6th Man of the Year in the NBA is an award given to, essentially, the most effective bench player in the league. Obviously, a lot of these guys would be capable starters in the league, but find themselves leading the 2nd unit on the floor.
They also spend time with the starters as the first player off the bench to replace someone in the lineup. This award has been held by a number of great players, with only a handful of people having won the award multiple times.
The Dallas Mavericks have never had anyone win the award more than once, but have had three players hoist the trophy in team history.