as noted in the comments sard's theorem isn't really used in this way, but here is an example of the sort of thing it is about. for $f:\mathbb{R}^2\to\mathbb{R}, f(x,y)=x^2+y^2$ we have the derivative zero only at the origin (the set of critical points), and $f(0,0)$ (the set of critical values) is just a single point, which has measure zero in $\mathbb{R}$. for any other $r>0$, $f^{-1}(r)$ is the circle of radius $r$, a manifold of dimension $1=2-1$. the theorem is used often to say you can find (lots of) points like $r\in(0\infty)$ that have nice preimages.