An expression of the form $a_nx^n+ a_{n-1}x^{n-1}+\ldots +a_1 x+a_0$ can be interpreted in two ways:
(a) The $a_k$ $(0\leq k\leq n)$ are certain constants taken from a "ground field" $R$, e.g., $R:={\mathbb R}$, and $x$ is a variable taking values in $R$, so it makes sense to compute powers $x^k$. All in all such an expression defines a function $p:\quad R\to R,\quad x\mapsto p(x):=a_nx^n+ a_{n-1}x^{n-1}+\ldots +a_1 x+a_0\ .$
Example: If $n=2$ and $a_0=3$, $a_1=0$ and $a_2=7$ then $p(x)=7x^2+3$. If $R={\mathbb R}$ then one can graph the function $x\mapsto p(x)$ in the usual way.
(b) The $a_k$ $(0\leq k\leq n)$ are certain constants taken from a "ground field" $K$, $X$ is a "variable taken from nowhere", called an "indeterminate", and one considers the "formal polynomial" $p(X):=a_n X^n+ a_{n-1} X^{n-1}+\ldots +a_1 X+a_0\ ,$ a purely algebraic entity which is completely determined by the coefficient vector $(a_0,a_1,\ldots, a_n)$. A standard task is then to construct a larger field $L\supseteq K$ such that the polynomial $p(X)$ factors as $p(X)=a_n\ \prod_{k=1}^n(X-\lambda_k)$ for certain $\lambda_k\in L$.
Concerning calculus the following is important: A given (real or complex) coefficient vector $(a_0,a_1,\ldots, a_n)$ defines the corresponding function $x\mapsto p(x)$ uniquely. But the converse is also true: A given polynomial function $p(x):=a_nx^n+ a_{n-1}x^{n-1}+\ldots +a_1 x+a_0$ cannot be realized by some other coefficient vector (a_0',a_1',\ldots, a_m'). The reason for this is that one has $a_k={p^{(k)}(0)\over k!}\qquad(k\geq0)\ .$