The Conditional Stellar Mass Function (CSMF, also called the Conditional Mass Function or CMF) is a parameterized, unnormalized Probability Density Function giving the average number of galaxies with stellar masses within a range interval within a halo of a given mass, i.e., the number of galaxies with stellar masses within range (m +/- dm)/2 that reside in a halo of mass M. It is a concept, and the actual function is determined through compiling data, and mathematical formulae are heuristically devised to match it and/or devised based on physical arguments. It is modeled after the Conditional Luminosity Function.
A CSMF is an element of a Halo Model, a model that explains the galaxies in Dark Matter Halos, relating them to Dark Matter distributions produced by Cosmology simulations, presumably to match observation of large Surveys such as Sloan Digital Sky Survey.
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.