First of the alchemical trump cards the Magus represents the element of Mercury. As Crowley says, "Mercury represents action in all forms and phases. His is the fluidic basis of all transmission of activity and on the dynamic theory of the Universe he is himself the substance thereof."
Skill, cunning, craft. The Magus is making his world, changing and shaping it according to his Will. That is how powerful he is and you are - you can create your world. Be sure you know exactly what you want though because that is what you are going to get.
A quote from the movie What Dreams May Come is perfect for this card: That which you believe becomes your world.
My Tarot Card Practice. I have been studying Tarot for over 20 years now and am still learning new things daily.