From my perspective, a heavy ball is one that has deceptive speed because of spin. In other words, it comes at you and hits your racquet harder than you expect because the spin pitches it at you faster than it seems it should from watching it prior to the bounce.
I've created this by playing with a ball machine and turning the topspin way up. You can see it come over the net, bounce, and then all of a sudden it seems to accelerate towards you. You definitely can learn how to time a heavy ball if you realize it is coming, but I can imagine that the way Nadal's forehand must seem to jump off the court would be quite intimidating.