The baby feels what his mother means by how she talks. The baby will only understand what she says when he is older. Babies first learn to understand the form, the nonverbal, the how, and only later grasp the contents, the verbal, the what. Compare it to the way a dog can look you in the eye, wagging its tail, after you have said in a friendly tone of voice that it is a lousy dog. The dog understands the form of the message, not the content.