St. Thomas's argument is based upon the idea that there can not be an infinite regress of insufficient causes. There has to be a cause that is sufficient unto itself. This cause we call the "unmoved mover" because it is unique. It's existence and power is not dependent upon another. Now, if such a sufficient cause, such a unmoved mover, did not exist, there would be nothing. But there is something, in fact, an entire universe. Therefore there must be an unmoved mover.
This argument of course for many of us proves nothing about the nature of God. That he is a personal God that is personally interested in us, all this is based upon revelation and not reason. It is important to understand what traditionally we believed this argument proves and what it doesn't prove. Hope this helps.