I found this part in the book of Bernard Schutz on GR. (Sec 12.2: General Relativistic Cosmological Models).
Robertson-Walker metrics: We shall adopt the following assumptions about the universe: (i) spacetime can be sliced into hypersurfaces of constant time which are perfectly homogeneous and isotropic; and (ii) the mean rest frame of the galaxies agrees with the definition of simultaneity.
Can someone kindly explain the second assumption, in particular what is a "mean rest frame" for a galaxy.