There are many types of stars in space. Some of these stars are easy to recognize and always exactly the same brightness. If you were to put two of these stars side-by-side, it would be hard to tell them apart.
Stars like this are called Standard Candles.
Standard candles are important in astronomy. They allow us to measure distance. If you have two stars of the same type, but one is further away than the other, it looks fainter. Knowing that they should be the same brightness allows us to work out the distances between them.
There are different kinds of Standard Candle star. A type called Cepheid variable stars were used to show that the Universe contained many galaxies, not just our own. The most famous type today are probably type Ia supernovae. These allow us to measure the distance to galaxies, millions of light years away.