The red clump (RC) is a grouping of stars (red clump giants, RCGs, red clump stars, RC stars, or clump giants) which are in the same evolutionary phase as the horizontal branch (HB), but are clumped on the H-R diagram (HRD) rather than spread over a range of surface temperatures. Typical is on the order of 5000 K and between 10 and 100 solar radii. This is roughly at the cool end of the HB line, and shows as a bump on the red-giant branch on the diagram.
Metallicity appears to be the factor regarding whether a star remains in the clump: they are generally not low metallicity stars. It also depends upon their previous main-sequence spectral class
Red clump stars, like HB stars, have passed through the RGB (for which they needed a mass of at least half a MSun as main sequence stars) and have begun helium fusion (triple alpha process) within the core, with a surrounding shell of hydrogen fusion. After the red clump stage, when a core of carbon (product of the triple-alpha fusion) has accumulated to the point that the helium fusion is in a surrounding shell, they enter the asymptotic giant branch (AGB) stage.
Red clump stars have been used as standard candles, e.g., to determine the distance to the galactic center; to the extent that they occur in one spot on the HRD, their luminosity variation is limited.