The Hubble constant is a number that describes the rate at which the universe is expanding. It is named after the astronomer Edwin Hubble, who discovered that distant galaxies are moving away from us, and the farther they are, the faster they seem to be receding. This relationship helps scientists understand the scale and age of the universe. The value of the Hubble constant is measured in kilometers per second per megaparsec (km/s/Mpc). This means that for every megaparsec (about 3.26 million light-years) away a galaxy is, it appears to be moving away from us at a certain speed. Accurate measurements of the Hubble constant are crucial for cosmology, as they provide insights into the universe's expansion history and its ultimate fate.