The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack of will.

This quote means that the reason why some people are more successful than others is not because they are stronger or more knowledgeable, but because they have a stronger desire to be successful, or stronger willpower. A successful person will take action to reach their goals, while someone who lacks willpower may give up too easily, not take advantage of opportunities, or not try hard enough.