A Standard Candle is any type of astronomical object whose brightness is known or can be determined irrespective of distance. When an instance is discovered, its distance can be determined by its brightness, as well as the distance to objects determined to be near it. For example, a Galaxy's distance can be determined if a Type Ia Supernova is observed in it because the Supernova's brightness relates to how long it lasts, so by timing it, its brightness can be determined, and by how bright it is seen from Earth, its distance from Earth. Standard Candles are keys to measuring Cosmic Distance Ladder.
A number of standard candles have been identified:
Cosmic Distance Ladder
Tully-Fisher Relation (TFR)
Type Ia Supernova