For realists scientific progress is progress towards knowing the truth about the nature of reality.  This can be quantitative accumulation of more truth or it can be improving the degree of approximation in approximate truth.  While no realists want to claim that any particular theoreis are "the truth" (modern realists are all fallibilists) they do very strongly insist that the degree of approximate truth is "converging" on "the truth" (perhaps assymptotically, forever getting closer but never reaching, or reaching it in an "infinite amount of time").