Hey mathlover10.

They are just dummy variables: my guess is that he is trying to avoid using the variables t, t0, y and y0 for the limits.

Remember that Integral 0 to x f(x)dx is a lot more complicated than Integral 0 to x f(s)ds. The second one is usually what is meant but sometimes you can get situations resembling the first (The Fresnel integrals come to mind).