Imagine that you have a type of star that is easy to recognise and is always exactly the same. In other words, if you were to put two of these stars side-by-side, it would be very hard to tell them apart.
Stars like this are called Standard Candles.
A good use for standard candles comes in when you have two stars of the same type, but where one is further away than the other. Although they are the same, the more distant one will look fainter. Knowing that they should be the same brightness allows us to work out the relative distances between them.