To play good, high-percentage basketball you have to know what the defense is trying to do to you. It is not simply a question of whether they are playing a man-for-man or a zone, or changing defenses, but how they are playing whatever defense they are playing.
If you pass the ball and cut through, and no one follows you, then regardless of how sure you are that they are in a man-for-man, you had better not keep on going. If no one goes with you, find out immediately why not. If he fell, you can go on to the basket and get an easy layup. But if he went to double-team, you are of no value moving away from the ball. You should curl back toward the ball the moment you realize that no one has gone with you. You need to be an immediate outlet for the man with the ball. And there is no way you can do that unless you are constantly aware of what the defense is doing.
Reading the defense also applies to getting the ball to a cutter or to a big man posting low. You have to be sure you throw the ball away from the defense. This seems like common sense, but many good players just toss the ball without ever making an effort to throw it to a particular side, away from the defense.
From Dick’s book Stuff
Welcome To Keys To The Gym 
Players told by coaches “run the play as diagrammed” can’t do this. It’s not exactly the same was what Dick brings up here, but the necessity of reading the defense and responding accordingly is one reason I like teaching Torbett’s “read and react” offense. The very best players, of course, are pre-reading the defense and knowing how to attack before they get the ball.