Bob and Marin are discussing ploughing - and how to plough - but really it's an analogy on making mistakes, and how to learn from them. When you're ploughing, you need to keep your eyes ahead of you all of the time to make a straight furrow - when a man's ploughing, he doesn't have any time for looking back. If you make a mistake, you can't correct them and must keep going - always learning from the mistake, so as not to make the same mistake again. Gradually, you plough more of the field and move farther away from the mistake, and you learn to not fret about it anymore. Pretty soon you can't even see the mistake anymore - but you never forget the lesson it taught you...