Ejaculatory fluids of the male human. Can be traced back to the mid-70's to David Bowie's alter ego Ziggy Stardust. In the example, "white light" describes the high of freebasing heroin.
White light - White light gonna drive me out of my brain
White light - White light gonna make me feel so insane
White heat - White heat shapin' them down to my toes
White light - White light's got it now, goodness knows