A device for a baby or small child to suck on at times when they are not being fed. Presumably the action of sucking soothes the child even when they're not getting any food out of it. Seems to make it a little easier to wean a child if my younger siblings are any example, but as the kid gets older they start looking silly running around with a pacifier in their mouth. According to http://parentsplace.com/health/kidsdent/qa/0,3435,6835,00.html, though, a pacifier is better for the mouth/teeth than thumb sucking (and easier to get rid of when the child gets older).