Cepheid variables are types of pulsating supergiant stars characterized by periodic changes in their apparent brightness. These pulsations are caused by the cyclical heating and cooling of the star's outer layers, leading to a regular expansion and contraction. The primary importance of these stars in astrophysics stems from the established period-luminosity relationship: the longer the measured period of the pulsation, the greater the intrinsic luminosity of the star. Because this relationship allows for the accurate estimation of distance using the apparent and calculated absolute brightness, Cepheid variables are crucial astrophysical tools used to determine distances within galaxies, effectively serving as 'standard candles' in cosmology.