This quote is saying that there are a lot of negative qualities that we all have, but we might not always be aware of them. These include things like egoism, arrogance, and selfishness. If we can learn to recognize and overcome these qualities, we will be better equipped to face the challenges of the world.
By mastering these "invisible enemies," we will be better able to deal with the "visible enemy" - the challenges and obstacles that we face in our daily lives. We will be more resilient and better able to cope with adversity, because we will have learned how to overcome our own negative qualities.
Ultimately, this quote is about the importance of self-improvement. By recognizing and overcoming our own flaws, we can become better people and better equipped to deal with the challenges of life.