I have a science assignment which asks me how the diagram is used by astronomers and why it is useful. The internet just has really confusing and long winded explanations, which are no help!
The H-R diagram can be thought of as a graph, with color (temperature) on the x-axis and luminosity on the y-axis. Since stars go through a definite "life cycle" which can be mapped on the H-R diagram, a star's location on the diagram can indicate both its mass and its relative age.
The HR diagram and associated isochromes tell us the evolutionary paths of stars through their lives. This can help us determine, for example, the age of a star cluster by the positions of the stars on the diagram.
