The Five Dhyani Buddhas are, starting at the top left and moving clockwise: Ratnasambhava, Amitabha, Vairochana, Amoghasiddhi and Akshobhya. They are not historical figures, like Gautama Buddha, but they are celestial buddhas; transcendent beings who symbolize universal divine principles or forces. They represent various aspects of the enlightened consciousness and are healers of the mind and soul. Vairochana is regarded as the first Dhyani Buddha in Nepalese-Tibetan Buddhism. He represents the cosmic element of Rupa (form). Since he is the first and considered the Primordial Buddha, he also sits in the center of the Mandala. His mantra with seed syllable is: Om Vairochana Hum.
A mandala is a sacred consecrated space where no obstacles, impurities or distracting influences exist. A mandala is no mere image of heavenly power. Buddhists believe a mandala is the receptacle of the holy power it portrays. Its purpose, and the goal of every one of its symbolic images, is to help the meditator realize the divine power within himself and achieve his own inner perfection.
Meditate on this Mandala, give Vairochana's mantra and the other Dhyani Buddha mantras and you will go far young Padawan.