A pattern mapper is a program/formula which can accurately predict what cards will come on the turn and river (among other things). It was first discovered (or at least revealed) by a poster known as Ulysses.
It all started with
this original pattern mapping thread starting with about the 11th post in.
It then spread to RGP and other places. Before you get into an uproar check the FAQ for the Internet forum - there is no such thing as pattern mapping, it was just a joke that got a little out of control.