What is the Hubble constant essentially a measure of?
Recession (expansion) speed of the space in-between two galaxies.
As of now, the Hubble constant is nearly 71 km/sec/mega parsec.
A quasar is what appears as a point source of light, in a far distant
galaxy. Perhaps, it is the Black Hole of the galaxy. Measurements
reveal that this source is receding from the Earth (of the
galaxy Milky Way). This is how Hubble constant is estimated. A
generalization,based on recession of different quasars from Milky
Way, leads to the notion of expansion of our universe.