Catabolite Repression is a biological process that allows microorganisms to develop and adapt to new sources of energy (Wikipedia). This is important because many new bacteriological environments have differing sources of energy and carbon; some are not metabolisable, and need to be processed differently.
Basically, bacteria and other uni- or multi-cellular organisms need to process energy to survive; they absorb and break down energy sources based on their specific needs. When a new energy source appears that is preferred by the bacterium, it is able to change its own biological makeup to exclude other sources of energy, thus allowing it to focus on the better source. When a bacterium enters a new environment, it must find the best source of energy; it can rapidly exclude sources until the best one is found. This is done by "switching off" the enzymes that process various energy sources, leaving only the preferred enzymes "on."