Vajrasattva is the sovereign lord of all the mandalas of the Vajrayana, or Diamond Vehicle. To meditate on Vajrasattva is the same as to meditate upon all the Buddhas. His hundred-syllable mantra is the quintessence of all mantras.
The main obstacles to progress along the path to enlightenment are the obscurations that come from our past negative actions. There are various kinds of negative actions. Some, like killing, stealing, lying or cheating are obviously intrinsically unvirtuous. Others are transgressions of vows and precepts that the Buddha or one's teachers have taught to help one progress spiritually.
The main purpose of Vajrasattva practice is to purify these obscurations. It is said, "The only virtue of sin is that it can be purified." In fact, there is nothing that cannot be purified, even the most apparently heinous deed.
The four powers
In order to purify negative actions completely, one needs four powers or strengths: the power of support, the power of regret, the power of the antidote, and the power of resolution.
Purification requires a support for us to express our remorse, make our confession and repair the effects of our past negative actions. In this case, the support is Vajrasattva. Visualise him above your head, utterly peaceful and smiling, brilliant white like a dazzling snow mountain illuminated by the rays of a hundred thousand suns. He is sitting in full vajra posture, upon a thousand-petalled white lotus and a moon disc. In his right hand he holds a golden vajra at his heart centre, and in his left, a silver bell resting on his hip. Vajrasattva is wearing the thirteen Sambhogakaya adornments - the five silken garments and the eight jewelled ornaments. He is in union with his consort, Vajratopa, who holds a curved knife in her right hand and, in her left, a skull cup filled with amrita, the nectar of immortality. Visualise Vajrasattva not as someone made of flesh and blood, but like a rainbow in the sky, vivid yet empty. Unlike a rainbow, on the other hand, he is not simply something perceived physically, for he is pervaded with the wisdom and compassion of all the Buddhas. Think of him as your kind root teacher, appearing in the form of Vajrasattva.
Next week will continue the information you should know before doing a Purification with Vajrasattva. This is one of the first practices I participated in when finding Buddhism. I hope it helps you as much as it helped me. Namaste', in metta.