AstroJon

Cephid variables have a period that is related to it's brightness. If you know the period, you know how bright it actually is. You can compare how bright it actually is with how bright it appears giving you an estimate of the distance. Essentially we need to know how bright an object actually is, whether it is a cephid variable, supernova, quasar or whatever and compare this with how bright it appears. This link might help http://en.wikipedia.org/wiki/Cosmic_distance_ladder