Up top is a term in basketball that relates back to the key on the basketball court above the free throw line. A ball is reversed "up top" and a shot is taken from "up top", or at the top of the key area. It can also extend to anywhere above the three point line that is not in the corners in any way. So, up top is more of a slang term that could mean a few different things pertaining to the basketball court.
If the offense has the ball up top it is one of the more dangerous areas to attack from on the court because of the many options that can come from this area. A jump shot, a drive to either side of the court, a screen play, or a reversal either way could all happen making the area difficult to defend well which bodes well for the offense. If the ball is moved up top it is usually in the hands of the point guard or a guard who can handle the ball well too which can leave the defense vulnerable if they fail to rotate fast enough.
The defense must contain the middle of the floor, especially the top of the key and beyond area to successfully stop the opposing offense. Good rotations on ball reversals and staying in front of any player up top is essential because he/she can move the ball in so many different directions or ways from that spot. All in all, good solid defense is needed up top just as the case with everywhere on the court.