A lullaby is a soft and soothing song usually sung to an infant or small child. Many have repetitive lyrics. But lullabies are much more than a baby song as well.
They are an intimate connection between the singer and the baby. They let the baby know that all is well, that there is someone there to care for them, and that they can sleep. They are reassurance of security and a wonderful way to signal that it is now time to sleep or be calm.
In other words, they are a form of communication that sends a message much more than the simple song itself. They are expressions of love and security that are understood long before the words have meaning to the infant.