Well, that means is that as long as the parameter that measures the hardness and in this case, it's the size of a vertex cover for your graph, as long as that parameter is small or fixed, then you can solve that problem in polynomial time and that is what this here says, so as long as the parameter is fixed, the problem is tractable, if the parameter is not fixed, so for example, there's a function of n, then the problem is not tractable. 