What information do microarray image convey?
Each spot overlaps with an oligonucleotide probe, that is designed to identify a specific nucleic acid sequence. Gene expression arrays have probes which are complementary to sequences derived from the exons; therefore they will hybridize with these sequences. 

Before hybridization the cDNA (in this case) is labeled with a fluorescent molecule such as Cy3 or Cy5 (two dyes can be used to differentiate treated and control). Note that each spot will have many probe molecules. 
Fluorescent intensity is proportional to number of hybridized DNA strands, which in turn is proportional to its expression in the sample.

Extensive amount of texts are available on microarray data analysis which you can find quite easily.