Sigmund Freud hypothesized that the unconscious understanding of sexual desire started as early as birth, allowing the libido to go through stages where different parts of the body bring pleasure. These are called Freud's Stages of Psychosexual Development.
In infancy, the libido is in the oral stage, as feeding is one of the only conscious desires an infant understands. The oral stage focuses on the mouth and the consumption of food for pleasure, and typically lasts until the infant's eighteenth month, when it is replaced by the anal stage. The oral stage is marked by constant use of the mouth for exploring the world, placing things in the mouth and chewing on them, and generally checking items for edibility, as this is the main source of pleasure in the infant's libido. The oral stage develops as the infant nurses and derives pleasure from its oral connection with its mother, and slowly switches as the infant is weaned. The "psychosexual" aspect of this is specifically not "sexual" in the adult sense, but in the sense that the unconscious is deriving pleasure from oral activities.