When one unites the mantra, the guru, and the deity, the mantra awakens. The mantra is a mystical sound. The guru is the giver of knowledge. And the deity is the experience of that knowledge. The guru leads the disciple from dark ignorance to luminous knowledge by means of the mantra.
Here is an example: Suppose I tell you that there is a ghost in that banyan tree by the side of the road. After you hear this from me, you begin thinking about the ghost. You also know the meaning of the word ghost: a terrible, frightening spirit. One dark night you need to pass near that tree on an errand. All of a sudden, you remember the story of the ghost that I graphically described to you. As soon as you go under that banyan tree, your hair stands on end, your throat becomes dry, and you have terrible palpitations. As you look at the tree, a branch moves a little in the wind, and you either cry out ‘Ghost! Ghost!’ or fall unconscious. This is the experience of the ghost, which came from the awakening of the ‘ghost’ mantra. The word ghost is like the mantra; I am the guru who described that word to you; and your experience that there is a ghost in that tree is the deity of that ghost mantra.