Hertzsprung-Russell (H-R) diagram shows the relation between temperature of stars and their luminosity.
It shows the position of stars in its life-cycle. Star takes birth in nebula and moves to main-sequence. When the fuel finishes, it forms red-giant or super-giant depending on the mass of the star. The red-giant moves on to become white dwarf and super-giant goes supernova and forms neutron star or black-hole.
Most of stars in the H-R diagram are main-sequence stars as majority portion of their life-time stars spend sin main-sequence.
