The Schönberg-Chandrasekhar limit is the maximum stable isothermal mass of the helium stellar core formed from fusion in a main sequence star. A core is isothermal (same temperature throughout) if the energy source (fusion) surrounds it (i.e., forming a shell) and everything is sufficiently stable that the core falls into and essentially remains at equilibrium. Only stars in a certain mass range grow such a core large enough to reach the limit. For those that do, helium fusion begins and the star enters the red-giant stage.
The limit depends upon factors in the star:
Mcore/M ≈ 0.37(μenv/μcore)²