Velocity proportional to distance
Hubble's law states that a galaxy's recession velocity is proportional to its distance from us: v equals H-zero times d, where H-zero is the Hubble constant. Edwin Hubble published the relationship in 1929, building on Vesto Slipher's redshift measurements and Georges Lemaitre's earlier theoretical derivation. The further away a galaxy is, the faster it appears to recede.
The constant is measured in kilometers per second per megaparsec, with current estimates clustering around 67 to 74, the spread being the ongoing Hubble tension between different measurement methods.
Expanding space, not motion through space
The recession is not galaxies flying through space from a central point. Space itself is expanding, stretching the wavelength of light in transit into a cosmological redshift. A useful analogy is dots on an inflating balloon: every dot sees every other dot receding, with no dot at the center. Running the expansion backward points to the Big Bang, and the inverse of the Hubble constant gives a rough estimate of the age of the universe.