What is a standard candle?
A standard candle is an object that has the same luminosity, or energy output, wherever it appears in the universe. Imagine if every blinking red-colored flashlight bulb were exactly 100 watts, no matter where it is seen; in that case, viewing this light at night can tell a person how far away that flashlight is by measuring how bright the red light appears.
Regrettably, not many bright objects in the universe are standard candles. Red stars, for example, can have very different luminosities. It is extremely important, therefore, to find very luminous objects that serve as standard candles, so we can measure distances to faraway objects in the universe that are too distant to be measured using parallax.