H₀, or the Hubble constant, is a value that describes the rate at which the universe is expanding. It is measured in kilometers per second per megaparsec, meaning that for every megaparsec (about 3.26 million light-years) away from us, galaxies are moving away at a certain speed. This constant helps astronomers understand the scale and age of the universe. The determination of H₀ is crucial for cosmology, the study of the universe's origin and evolution. Different methods, such as observing distant supernovae and measuring the cosmic microwave background radiation, can yield varying values for H₀, leading to ongoing debates in the scientific community about the universe's expansion rate and its implications for dark energy and the Big Bang.