This is an advanced statistic used to determine how much a player contributes to a given win. The statistic takes into account the various things a basketball player does to win or lose a game, and weighs them appropriately to provide a Win Share. Essentially, this statistic takes into account everything a player does and converts it to a percentage of wins for the team.
The statistic is based on 48 minutes because that is the exact length of an NBA game. As such, the statistic can measure how much the player contributes to a winning effort on a per game basis based on their per minute performance. The idea for win shares comes from famous baseball sabermetrics expert Bill James and was perfected for basketball by Dr. Dean Oliver in his book, "Basketball on Paper." The formula is a bit complicated, but takes into account a player's offensive and defensive efficiency as well as their points produced.