Distance for Non-Euclidean Space

Question

In the book "In pursuit of the Unknown" by Ian Stewart, page 19 of chapter "Pythagoras' Theorem" shows the equation for the distance between two points in a non-euclidian space from point $(x,y,z)$ to $(x+dx,y+dy,z+dz)$ as:

$ds^2 = X dx^2 + Y dy^2 + Z dz^2 + 2U dx\,dy + 2V dx\,dz + 2W dy\,dz$

Being $ds$ the distance between the two points, $dx$ the distance between $x$ and $x+dx$, and so on.
Could you clarify the meaning of $X, Y, Z, U, V, W$ in this formula?
Thank you!