The chirp mass (Mc or ℳ) is a function of the masses of two orbiting bodies that is related to the rate of energy transferred away through gravitational waves. Its role is somewhat like reduced mass: it encapsulates some useful calculation and constitutes a relation between masses that some observations reveal.
Mc = ( m1 m2 )3/5 / ( m1 + m2 )1/5
The following differential equation approximately relates this value to the orbit's GW frequency and its rate of change:
Mc = c³/G(5/96π-8/3f-11/3df/dt)3/5
This is the equation solved for Mc in terms of f and df/dt, the latter two being the values that can be measured from observation. (I assume the name is chirp mass because these two values are evident from a GW chirp.) It is approximate because it is based upon the first term of a multi-term equation modeling the chirp's frequency increase. Other mass-related quantities associated with GWs:
These can be determined by higher order terms of the same equation, and the accuracy of the determination is lower. Because of known relationships between the chirp mass and these quantities, the individual masses of the two bodies can be determined (to some limited accuracy) from them.