An infant is a baby. When you were an infant, you weren't able to walk, talk, or feed yourself (but you had a pretty mean burp).
Infant is most often used for a very young human, but you can also use it to talk about animal babies. People are considered infants from the moment of birth until approximately one year old, an age when many babies are starting to walk and talk. At this point, they're commonly called toddlers instead of infants. The Latin root is infantem, which as a noun means "babe in arms," and as an adjective "unable to speak."