The Hubble Constant is a number that describes how fast the universe is expanding. It is named after the astronomer Edwin Hubble, who discovered that distant galaxies are moving away from us. The constant is usually expressed in kilometers per second per megaparsec, meaning that for every megaparsec (about 3.26 million light-years) away a galaxy is, it moves away from us at a certain speed. Scientists measure the Hubble Constant to understand the rate of expansion of the universe, which helps in estimating its age and size. Different methods, such as observing supernovae and cosmic microwave background radiation, provide varying values for the constant, leading to ongoing research and debate in the field of cosmology.