For a wood fire, t is the lifetime

For a wood fire, t is the lifetime of the fire, M is the mass of wood, and L is the luminosity (brightness) of the fire. For a main sequence stars, t is its main sequence lifetime, M is the mass of the star, and L is its luminosity.
For a wood fire, any combination of mass of wood and brightness of fire is possible. Recall from Chapter 16, however, that the luminosities and masses of main sequence stars are closely related to each other by the mass-luminosity relation found from observations of binary stars. The mass-luminosity relation can be expressed as