Vajrasattva’s name means “diamond being” and he represents the innate purity of the mind. As the article says,
You can imagine your mind as being like a sky through which clouds pass. The clouds come and they go, but the sky remains untouched. The sky is inherently blue and clear, and although its blueness and clarity can be obscured it can never be destroyed. The clouds are like the greed, hatred, and delusion that pollute the mind. Because of the transient nature of these mental states, they cannot be said to be an inherent part of the mind. They may obscure the mind’s inherent awareness and compassion, but those qualities are never absent.
The 100-syllable mantra is an evocation of the path to full awakening, from the first invocation of Vajrasattva as a protector, to the wish to save all sentient beings.