It's simply a simple graphic, not intended to be representative of anything in particular.
Well...the greys of the site are chosen specifically as a metaphor for what we cover being "shades of grey" - ie, nothing as "black and white".
The header graphic is about to change very soon - the current header looks like a rushed late night job (which it more or less is

) and a new once is about to come into effect, which will remain simple, but hopefully be better presented.
The yellow/gold I guess could be argued to represent the light of enlightenment, but my personal consideration is simply that it adds aesthetically to the site.