What role does an infant's attention to facial expressions play in his or her understanding of language?

The infants only rely on their understanding of the facial expression of the people who live around him and how they speak or basically their language. Soon the infants keep noting the similar expressions on a daily basis and they try connecting the dots and see the possibilities. IIn this way they become attention seekers and they either try to copy the expressions or they smile/laugh or become excited on seeing such behaviors.