don't forget what comes after it:
SDL_BlitSurface( source, NULL, destination, &offset );
if you read this
you will see that the rectangle named "offset" is used to pass the position of the surface you want to blit.
edit: so it is simply the position of the surface you want to draw. if i was lazyfoo, i'd have called it "position" instead, haha.
edit: offset makes sense too. it is an offset to the upper left corner of the background surface you want to draw on.