The sketch summarizes the evolution of stars. The star leaves the main sequence when the hydgrogen in its core is depleted and turns into a red giant. The helium flash occurs when its core temperature exceeds 100 million K. Helium burning keeps it in the instability strip, where it often shows pulsating behaviour.
Eventually all sources of nuclear energy are depleted, and the star contracts into a white dwarf that cools slowly and gets fainter, as indicated by the downward moving curve.