A star passes through stages of instability and stability. When core hydrogen fusion begins in a pre main sequence star, the star enters a stage of stability.
When the body has sufficient mass, the collapsing gas and dust burn hotter which eventually reaches temperatures enough to fuse hydrogen into helium. Thus, the star turns on and becomes a main sequence star, powered by hydrogen fusion.
A pre main sequence star is a star in the stage when it has not yet reached the main sequence. Earlier it is a protostar that grows by acquiring mass from its surrounding dust and gas.
Hence, in the pre main sequence star stage, core hydrogen fusion begins and the star enters a stage of stability.
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