HR diagrams are used to place stars in different catagories based on their proporties. Luminosity is on the Y axis and temperature is on the X. The 4 categories are main sequence, red giants/super giants, blue supergiants, and white dwarfs. For main sequence stars the brighter they are the hotter they are. In the top left corner you have the hot a bright stars. In the bottom left you have dim and hot stars. In the top right you have the bright and cool stars. In the botto, right you have the dim and cool stars.