If you can dream - and not make dreams your master;
If you can think - and not make thoughts your aim;
If you can dream and make use of your dreams, then it is good to dream, but if all you do is dream, and nothing ever comes of your dreams, then you have allowed your dreams to master you or control you. Some people only dream. They never work hard to make their dreams come true. They allow their dreams to consume them. Dreaming becomes a way of life, but their dreams never come true. That is when the dream is the master. The dream is in control.
It is the same way with thoughts. Some people think on thoughts and spend their life in a day dream, thinking on thoughts that never bring about results.
Truly, some people dream too much. All they do is dream. They start working on one dream, and, before they can make it come true, they are dreaming again, and then working on that dream. Unfortunately, they never make any dream become a reality.
When you have a dream, you have to set goals to make that dream come true. If all you ever do is dream, none of your dreams will ever come to past. You will spend all your time dreaming with no manifestation of your dreams. Ultimately, too much time is wasted dreaming. That is when dreaming has become a negative factor in your life. The dream has become master over you.
If a man can dream, can remain committed to the pursuit of a creative idea, of a creative quest without becoming a slave to one single idea, then Kipling applauds this man. He says this man will have "the Earth and everything that's in it."
A person can make his dream master by giving the dream power, by allowing the dream to steer the person (the ship) through life (the sea).
If a person empowers a dream, then his sight of endless possibility, his sight of the world may become narrowed or funneled into trying to force his dream to become real. His creative nature will then, ironically, become less creative, less unbounded, and will instead be stunted due to the energy and focus it will take that person to make that dream come true. Thus, the dream has now become master of the man.