For binary stars, the mass function (or binary mass function) is a formula of observable values that yields constraints on the masses of the two stars if the radial velocities can be determined through Doppler shifts.
P ——— V³ 2πG
This value is equal to the following constraint on the masses:
m³ —————— sin³i (M+m)²
The inclination is evident if it also transits, or may be estimated by the "average" inclination. For star population studies, there is the potential that some useful statistics may be derived assuming the inclinations are random.
Note that the phrase mass function is to indicate it deals with physical masses. The two-word phrase is used in probability (probability mass function) for an incompatible concept: a function yielding the probability of some discrete random variable taking on a given value.