A pacifier is a soothing device for babies that's made of rubber, silicone, or plastic. If you can picture the cartoon character Maggie Simpson, then you can picture a pacifier.
Most infants are born with a strong need to suck, and many of them suck their own thumbs or fingers to soothe themselves. A pacifier works the same way. While some parents and even doctors worry that continuing to use pacifiers too long can damage babies' developing teeth and palates, such problems are actually very rare. The original meaning of pacifier is "anything (or anyone) that pacifies," from the Latin pacificare, "make peace; calm."