This is the player who is regularly used as a team's first substitute and/or the best non-starter in a team.
Just because the sixth man doesn't start the game, that doesn't mean that he's the sixth best player in a team. Often, the sixth man is good enough to be a starter, but the coach feels that by bringing him in later in the game, he can exploit match ups against the other team's second unit. The sixth man is often in the game during crunch time.
The most valuable sixth man of the season is given the Sixth Man of the Year Award. In order to be eligible, the player has to come off the bench in more games than he starts. Some of the best sixth men in the history of the NBA include Manu Ginobili, Kevin McHale (early in his career), Toni Kukoc, Lamar Odom, and Detlef Schrempf.