Partial derivatives allow us to approximate functions just like ordinary derivatives do, only with a contribution from each variable. If $x \approx a$ and $y \approx b$, then
$$f(x,y) \approx f(a,b) + \Bigl(f_x(a,b) \Bigr) (x-a) + \Bigl(f_y(a,b) \Bigr) (y-b).$$ This is sometimes written as $$df = \frac{\partial f}{\partial x} dx + \frac{\partial f}{\partial y} dy.$$
And, just as in one dimension, we can use higher derivatives to get a more accurate approximation.
Taylor Series in two variables:
$$f(x,y) = \sum_{n,m=0}^\infty c_{n,m} (x-a)^n (y-b)^m,$$ where $$c_{n,m}= \frac{1}{n!m!}\frac{\partial^{n+m}f}{\partial x^n\partial y^m}(a,b).$$