Protostar

A protostar is an extremely young star that is still within the process of gaining mass from the cloud that produces it. For a low-mass star like the Sun(or lower), the protostellar phase lasts about 500,000 years and is the first phase in the process of stellar evolution. This starts with the formation of a protoplanetary disk around where the protostar forms. This disk is essentially just a composition of gas and dust orbiting the star. The star takes in material from this disk to keep gaining mass and forming in order to eventually reach a state of equilibrium.

When the protostar just forms, the gravity within the molecular cloud is dominating the thermal pressure, causing the molecular cloud to form a dense, clumped core with accreted material. For reference, accretion is the accumulation of mass through gravity by a massive object. However, this material needs to then become able to undergo nuclear fusion to become a proper star. Between being a dense core of material and being able to undergo nuclear fusion is the protostellar phase.

Protostars don't get their thermal energy from nuclear fusion as they can't fuse hydrogen yet. They get their energy from the radiation of the protoplanetary disks surrounding them instead. However, once the protostellar phase is complete and the surrounding disk is depleted, the star is now a pre-main sequence star.