A bib is a small piece of fabric that's tied under a baby's chin to keep food or milk from staining her clothes. If you feed a toddler without using a bib, you'll quickly learn how useful it can be.
A baby's bib is just one type — there is also the bib that makes up the top part of overalls or an apron, the section that covers the chest. Sometimes the corresponding part of a bird's body, a patch of color on the chest, is also called a bib. The word stems from the now-obsolete verb bibben, "to drink," which is thought to be imitative of the sound your lips make as they sip liquid.