An M-Type Star is of the M-Class, a Spectral Class for cool stars including Red Dwarves (M Dwarves) which are Main Sequence Stars, and Red Giants, which are beyond the main sequence. The common element is the Temperature, which is less than other main sequence stars. They all have very weak Hydrogen Absorption Lines.
Some characteristics of M Dwarfs (Mass, Radius and Luminosity as fraction of the Solar values):
Brown Dwarf (BD)
K-Type Star (K)
L-Type Star (L)
Transiting Exoplanet Survey Satellite (TESS)