A bobblehead is a little figure with a very big head that wiggles when you move it around. These figures mostly bear resemblance to popular characters in games, movies, in real life or other. An example of 'other' are those little dogs on dashboards in cars who 'headbang' when the car is moving.
In Fallout 3, it is an item that you pick up and boosts one of your skills or S.P.E.C.I.A.L. stats