The Web is loaded with information about Mandalas with various sites dedicated to them, so here I'll be short and broad... for more info, put your search engines to work.

"The Sanskrit word mandala means 'circle' in the ordinary sense of the word. In the sphere of religious practices and psychology it denotes circular images, which are drawn, painted, modeled, or danced" C.G. Jung
"They are illustrious designs, symbolic of the universe, and are often used in eastern mystical tradition as an aid to meditation." They are meant to represent spiritual oneness, unity and wholeness. There are many variations of the mandala, each carrying significant meaning, seeking to resonate within the "eye of the beholder."

As the viewer is absorbed in the structure, order & relation within the Mandala, the mind is allowed to move from the periphery and focus on the center, the self.