# How to understand gauge fixing condition?

In Peskin & Schröder's book, they wrote that there is no propagator for an Abelian field due to gauge invariance of the action, and then proposed a gauge fixing condition: $$\partial_\mu A^\mu = \omega(x)\tag{9.55}$$ in order to count each physical configuration once. Where $$\omega(x)$$ can be any scalar function.

I don't really understand what this equation means intuitively and how it actually helps counting each physical configuration only once?