Expression data - how to deal with it?

Hello dear forum
I am working currently with the expression analysis of various genes by Affymetrix microarray chips. Here I come to terms with which I unfortunately can not do much.

In one illustration the absolute expression is specified (unitless values between 0 and 2000), in the description they say, "normalization GCOS: TGT (100)".  GCOS is probably an Affymetrix software and TGT  a "target value". The measurements  are, as far as if I understand correctly, somehow normalized to make them comparable. Unfortunately, I find nowhere a description of how this works. What does an expression of, for example 500 tell me? is it high or low? and what is the difference between TGT (100) or TGT (200)?

I would very much like to understand the basics.  

Maybe someone knows of it or knows a link where I can read.

Best Regards
JAccy