I know next to nothing about differential forms, but I saw them being mentioned repeatedly on this site, so I went to Wikipedia to try to understand what a differential form is. Most of it is going pretty smooth, but there's a paragraph I don't understand:
Since any vector $v$ is a linear combination $\sum v^je_j$ of its components, $df$ is uniquely determined by $df_p(e_j)$ for each $j$ and each $p \in U$, which are just the partial derivaties of $f$ on $U$. Thus $df$ provides a way of encoding the partial derivatives of $f$. It can be decoded by noticing that the coordinates $x^1, x^2, \dots , x^n$ are themselves functions on $U$, and so define differential 1-forms $dx^1,dx^2,\dots,dx^n$. Since $\frac{\partial x^i}{\partial x^j} = \delta_{ij}$, the Kronecker delta function, it follows that
$ df = \sum_{i=1}^n \frac{\partial f}{\partial x^i}dx^i $
I don't understand the bit about the coordinates being functions themselves. What does that mean?