A high-mass protostar is a precursor to a star; it is a star in the nursery stage that is still fundamentally a high-mass cloud of gas. All stars must go through the nursery stage before becoming full-fledged stars. The nursery stage consists of passing through the nebula.
A protostar is not yet hot enough for fusion to take place in order to become a star. When the protostar's temperature exceeds 10 million degrees Kelvin, it is then hot enough to become a main sequence star. High-mass protostars become larger stars as they pass through the nursery stage. Then they become a main sequence star, a red giant and finally a supernova.