What does the power of thought mean?

On the much-written-upon subject of the power of thought, James Allen notes that his little volume—the result of meditation and experience—is not intended as an exhaustive treatise on the much-written-upon subject of the power of thought. 

It is suggestive, rather than explanatory—its object being to stimulate you to the discovery and perception of the truth that—

“They themselves are makers of themselves.”

by virtue of the thoughts, which you choose and encourage—that mind is the Master Weaver, both of the inner garment of character and the outer garment of circumstance, and that as you may have hitherto have woven in ignorance and pain, you may now weave in enlightenment and happiness.

—JAMES ALLEN