What does x mean in (1-x)A – xB composite materials, molar ratio, weight ratio or volume fraction?
Here, $x$ is a variable $(x\in[0;1])$ denoting mole fraction of a given constituent in a phase of variable composition (non-stoichiometric compound).
This is an algebraic way of denoting a given compound from the phase diagram for a solid solution: $x$ corresponds to its abscissa axis.

And no, volume fraction is not equal to mole fraction and is overall a very inconvenient variable to use, so it's usually being avoided.
For example, volume fraction would deviate for the different morphologies and allotropes of the same solid, which is not useful for the description of chemical composition, not to mention it's also temperature-dependent.