An n-th order Taylor polynomial is the first n terms of a Taylor series: Tn(x)=c0+c1(x−a)+⋯+cn(x−a)n.
Using first-order Taylor polynomials is nothing new. Back in first semester calculus, we called this approximation the linearization: f(x)≈L(x)=f(a)+f′(a)(x−a)=T1(x).
Note that the n-th derivative of Tn is a constant. Using Tn to approximate f ignores the fact that f(n) can change. In particular, T0 pretends that the function doesn't change; T1 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.