What a center is
The centers are the nine geometric shapes of the bodygraph, each in charge of its own area of life: thinking, speech, direction and love, willpower, instinct, emotions, life force, pressure and inspiration. Some centers are motors (they supply energy), others are centers of pressure, awareness or expression.
Defined or open
A defined (coloured) center works consistently — it's what a person can always rely on. An open (white) center takes in and amplifies other people's energy: this is where a person learns and grows wise, but also where they are most easily conditioned. Neither is better — they are simply different ways of living.