The Hertsprung Russell diagram displays the relationship between the mass and luminosity of stars.

The vertical axis represents the stars luminosity or absolute magnitude. Luminosity is technically the amount of energy a star radiates in one second,

The horizontal axis represents the stars surface temperature Usually this is labelled using the Kelvin temperature scale. Notice the direction of the temperature axis is reversed, increasing from right to left.

Most stars are on the main sequence. Small dim stars are at the bottom right and bright hot stars are at the upper left. Our Sun lies in the middle of the main sequence.

When a star leaves the main sequence towards the end of its lif it moves to the upper right, becoming a Red Giant or Supergiant. Later it might become a white dwarf or neutron star to the bottom left, or a black hole, which is not shown.