Basically Hypnopaedia is "sleep teaching". It is meant to condition the brain towards promiscuity, use of soma, and complete acceptance of society. Through subliminal messages, people are conditioned for the excesses of society via hypnopaedic slogans such as "a gramme is better than a damn". The use of hypnopaedia strengthens the conditioning and indicates the subversive nature of the state. Huxley is showing the readers that propaganda starts at birth and can occur even when we are unaware of it, as when sleeping. He reinforces the point that people are unaware of how influential the propaganda is by constantly having his characters quote "hypnopaedic phrases."
It is not totally successful because there are people, like Bernard and Helmholtz resist it.