in simpe words, double means

two players defend one player to trap him or force him to give up the ball. The problem is if you double, there out to be one player being uncoverd, as both teams have the same number of players. PG usually can dribble the ball well and pass to that undefended player. When Lin or any player is hot, the other team will have to take the risk to double him to force him giving up the ball.

Just my 2 cents, may not be very accurate.