What is the role of Brightness (Flux) measurement when estimating the distance to a light source?

It allows estimation of distance provided the intrinsic brightness is known

Brightness, quantified as Flux, refers to the amount of light energy arriving at the telescope per unit of time from a specific celestial object. While this measurement is distinct from determining color or composition, it is a crucial component in calculating distance, provided the source is calibrated against known standards, often referred to as standard candles. If an astronomer knows the object's true, intrinsic luminosity (how bright it actually is), they can compare that to how dim it appears (the measured flux). The inverse square law relating intensity to distance then allows the distance to be estimated, enabling mapping of distant galaxies.