Taurine is a vitally important amino acid for cats, necessary for proper digestive system, liver, and renal function among other things.
In a cat's natural diet, the necessary taurine would come from the heart, liver, and other internal organs of its prey. Commercial cat food, though, must usually be fortified with taurine, since the processing of those wonderful meat by-products tends to eliminate much of the natural taurine content.
Because taurine is so vital to the proper functioning of a cat's digestive system, a single meal without an appropriate taurine content can be enough to make a cat very sick, or even kill it. This is why cats should never be allowed to eat dog food -- though it may smell and taste good to the cat, dog food isn't specially taurine-fortified the way cat food is.