They vary their intonation, which makes their speech sound more interesting to their baby. This is turntaking. Their baby is intrigued by this stimulation and will look at the parent, maintaining eye contact, which encourages the parent to continue. When baby looks away, parents interpret this as loss of interest in the interaction or assume that the baby is tired and let them have quiet time to sleep. If the baby looks to an object in the environment, the parent will talk about that object, label it, and perhaps bring it closer or offer it to the baby.
Parents interpret their infant's behavior as meaningful and they respond appropriately.
With frequent repetition of these interactions, the infant begins to learn that they can affect their environment. The development of speech and language skills is a maturational process.