A doll is a toy that looks like a small person, often a baby. When kids play with dolls, they might dress them up, push them in toy strollers, and have pretend tea parties with them.
Some dolls are the size of real babies and are played with in a way that's a kind of practice parenting — like pretending to feed and bathe a doll. Other dolls are much smaller and represent people of varying ages, like a family of dolls that are played with in a doll house, sitting at little tables and sleeping in tiny beds. The word doll is also an old-fashioned, slightly offensive term for a woman, especially one who's young and pretty.