For 'primitive' and 'modern' people alike, the name of a thing is a source of power over it. Ancient peoples achieved mastery over the forces of nature by giving those forces names. For instance, by calling the thunderstorm 'Indra,' the old Brahmins established a mechanism and degree of control over it. This is also clearly reflected in our own experience. If someone side-swipes your car in the parking lot, you do not have any power over him or her if you do not know their name, but if you know the name of the person responsible, you can claim damages.
Names are, therefore, power.
In a sense, a name creates the reality of the object for which it stands. For example, when I say the word 'diamond,' in a sense the reality of that object is created for all of us. Recognizing this power of letters, names, and words, the Vajrayana employs them in the form of mantras in order to bring about a certain kind of reality. In the Vajrayana, the naive assumption of the power inherent in names that was characteristic of early human existence is replaced by a critical understanding of the way names and words work to create a particular reality. The way they work is through the power of the mind.