A low mass star (or lowmass star) is a star of little mass. Some classify stars as "low mass" if they are below 1/2 solar mass. Others label that a very low mass star, and take .5 to around 2 solar masses to be "low mass".
Lower mass means dimmer and a longer lifetime, and for stars below 1/2 solar mass, the lifetime is longer than the universe's current age. They are thought to be fully convective, with helium throughout, and thought not to become red giants, but to end up as helium white dwarves.
Stars on the order of the size of Sun do become red giants, then eventually white dwarfs.