What is meant by "genes involved in the stress response"?
The comment by @fileunderwater is apt: stress-response-related genes are genes that are activated during stress, such as exposure to heat. In bacteria, for example, there are heat-shock proteins that are involved in keeping the tertiary structure of proteins (folding/3D shape) intact during periods of heat exposure. Genes encoding these heat shock proteins are then called to be upregulated, and you would find them back in a typical micro array experiment. Hope this helps.