They cluster in particular parts of the diagram. Brighter main sequence stars are more massive. The more massive a star is, the brighter it is. Main sequence stars are relatively young . As a star uses up its energy sources and shifts to new energy sources, it will wander on the HR diagram.
Mature stars are stars which have left the main sequence .
When a star has exhausted all its fuel , it becomes a white dwarf . For more information on this topic, see Life Cycle of the Stars .