I will start with an example.
Consider components of metric tensor g' in a coordinate system
$$ g'=
\begin{pmatrix}
xy & 1 \\
1 & xy \\
\end{pmatrix}
$$
We can find a transformation rule which brings g' to euclidean metric g=\begin{pmatrix}
1 & 0 \\
0 & 1\\
\end{pmatrix}, namely...