Complex sentence: The comoving distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe, hence such that the comoving distances on average are not increasing with time.
Split into simpler sentences: The comoving distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe. As a result the comoving distances on average are not increasing with time.