A Cepheid variable is a type of star that pulsates in a regular cycle, causing its brightness to change over time. These stars expand and contract, leading to variations in luminosity that can be observed from Earth. The period of these pulsations is directly related to their intrinsic brightness, making them important for measuring distances in the universe. Astronomers use Cepheid variables as standard candles to determine how far away galaxies are. By comparing the observed brightness with the known intrinsic brightness, they can calculate the distance to these celestial objects, helping to map the structure of the universe.