In feudal times the farmer was the quintessential professional. He or she would study weather patterns, gather knowledge of the environment including soil types, they would plant, weed, prune, harvest and store seeds appropriately and would process the farm produce diligently. The same would apply to the animal keeper in terms of their meticulous and dedicated approach, meant at only achieving the best results. Because of this painstaking application, the farmer is never truly satisfied because he or she understands that something may go wrong at any moment and therefore they need to stay on their toes. Such a farmer may seem inclined to harass their workers or family members and appear to be always complaining. Our elders understood this to represent the diligence and dissatisfaction the professional faces if they truly want to maintain the highest of standards.
The proverb is used to pick out qualities of a professional and hence can be applied to encourage someone to never be content with their work but always seek to make it better. It can also be used inversely to dismiss feelings of satisfaction when one should be focusing on improving.