enough: take something you know about an object, like a mass, a size, or a brightness, then measure what the mass, size or brightness appears to be, and suddenly, you know how far away that object has to be.
Add in a measurement of the object’s redshift, and you can figure out not only what the expansion rate of the Universe is today, you can figure out the entire expansion history, and therefore what makes the Universe up. For practically all of the 20th century, we used brightness — or standard candles — exclusively. But new developments in both galactic surveys (like SDSS) and our understanding of dark matter and inflation has enabled us to use a new technique: baryon acoustic oscillations, or a standard ruler, for this task.
Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction