Papa works all the time. He don't never rest. And worse than that, he works inside himself. I can see it on his face. Like he's been trying all his life to catch up to something. But whatever it is, it's always ahead of him, and he can't reach it.
Robert describes his father in this way while he is talking to Mr. Tanner after they try unsuccessfully to breed Pinky to his prize boar. Mr. Tanner is very surprised by the amount of insight contained in the quote, as are we. Up to this point we are not used to seeing this sort of thinking and elaboration from Robert or from any of the characters. Brevity is one of the principles of the Shaker religion, and up to this point, everyone more or less abides by it. The depth of thought shows Robert's growing maturity and also reflects his education.
The meaning of the quote goes back to an earlier reference that Haven makes to his, "mission." Having a mission in life is another Shaker principle. When Haven explains to Robert that killing pigs is his mission, he is not telling the whole truth. Haven is constantly chasing financial stability for his family. Killing pigs is the means to achieving this mission but not the mission itself. He wants to own the farm and erase the family's debt so that he can give Robert a chance to do something more with his life than waste away on a farm. He is in a race against death for this goal and, unfortunately, does not make it. From a slightly different point of view, Robert is also racing to make sure his son is soundly a man before he passes away.