A standard ruler is an astronomical object for which the actual physical size is known.
By measuring its angular size in the sky, one can use simple trigonometry to determine its distance from Earth.
In simple terms, this is because objects of a fixed size appear smaller the further away they are.
Measuring distances is of great importance in cosmology, as the relationship between the distance and redshift of an object can be used to measure the expansion rate and geometry of the Universe.
Because space is expanding, there is no one, unique way of measuring the distance between source and observer.