So I am doing an online calculus course and saw this equation: $ Q(x+x^2+ x^3/2) $ where the lecturer mentioned that "at $x=0$, if there is a polynomial, the quadratic approximation to a polynomial at x equals 0 is just all the terms up to the quadratic term. And higher order terms are dropped."
The resulting equation is $ x+x^2$.
What is this higher order polynomial? And how does it work that the $x^3/2$ is dropped?
Thanks beforehand!