An $n$-th order Taylor polynomial is the first $n$ terms of a Taylor series: $$T_n(x) = c_0 + c_1(x-a) + \cdots + c_n (x-a)^n.$$
Using first-order Taylor polynomials is nothing new. Back in first semester calculus, we called this approximation the linearization: $$ f(x) \approx L(x) = f(a) + f'(a) (x-a)=T_1(x).$$
Note that the $n$-th derivative of $T_n$ is a constant. Using $T_n$ to approximate $f$ ignores the fact that $f^{(n)}$ can change. In particular, $T_0$ pretends that the function doesn't change; $T_1$ pretends that the derivative doesn't change; etc.
To approximate $f(x)$, we first need to find a point $a$ near $x$ where we understand the function well, then compute the first few terms of the Taylor series for $f$ around $a$, and finally plug $x$ into that polynomial.