As it was already said it is an essential amino acid responsible for the formation of carnitine, collagen and calcium absorption. You see it's important to give cats food full of micronutrients and different vitamins. For instance, popcorn isn't the best choice for them and about it you can read here. Although, it's not that dangerous but definitely it's not an everyday meal for cats.
It is an essential amino acid which helps cats (and dogs) to process the proteins their body requires. A deficiency (not enough) of L-lysine, can result in slow growth of the kitten/cat. It's a good thing :) ... My cats love Royal Canin!