To understand this, think about all the things that computers can do. At times, they can seem to do everything that a human can do, only better and faster. Of course, this is not completely true and I do not think that most people think that computers are truly superhuman.
Computers can do some things much better than people can. A simple search on a search engine can go through millions of pages of information to find the thing that you want in less than a second. No person could do that. Computers can defeat the greatest chess players in the world. Computers can complete computations that would take humans lifetimes to finish. In these ways, computers can seem to be superhuman.
Of course, there are many things computers cannot yet do. The field of artificial intelligence has not gotten as far as people have predicted it would get. Computers cannot truly think at this point and therefore I would not call computers superhuman.