Cepheid variables are a type of pulsating star that changes brightness over a regular cycle. These stars expand and contract, causing their light output to vary. The period of this brightness change is directly related to their intrinsic luminosity, making them important for measuring distances in space. Astronomers use Cepheid variables as "standard candles" to determine how far away galaxies are. By observing the period of a Cepheid variable's brightness fluctuations, scientists can calculate its true brightness and compare it to how bright it appears from Earth, allowing for accurate distance measurements in the universe.