From The Sims wikia:
The PlumbBob (frequently misspelled as “Plumbob”) is used in most titles of The Sims series (including the MySims series) to identify the character selected and to which the player can give commands. It is also used to show the mood of playable Sims. The PlumbBob has become an important symbol to the franchise, and it is widely recognized by players around the world as one of the game’s identities. Several game covers, logos, and special products feature PlumbBobs in their designs.