The target minutes option allows you to set a minute value for each player to indicate how much you want them to play per game at that position. In a normal game with realistic settings each player should play within a few minutes of the target setting. However, many factors can cause a player to play more or less minutes than you specify. These factors include foul trouble, injuries, etc.
Also keep in mind that in a regulation game there are 240 total minutes to be played (5 players X 48 minutes), so if your total target minutes exceed 240 then some players may play less than their target range. Also, if your total target minutes are less than 240 then some players may play more than their target range.