The Stone-Weierstrass theorem

The Stone-Weierstrass theorem tell us that:

If $f$ is continuous on the interval $[a,b]$, then for every $ϵ>0$ there exists a polynomial $p$ such that $|f(x)-p(x)|<ϵ\forall x$.

In other terms, every continuous function can be approximated by some polynomial with a certain level of accuracy. This is why in ML polynomial are used so much.