In the television series Buffy the Vampire Slayer, a Watcher is a member of a secret organisation (the Watchers' Council) devoted to tracking and combatting malevolent supernatural entities (and particularly vampires), primarily by locating and training individuals with the talents required to fight such beings and win.
Notable Watchers include Rupert Giles (Buffy's Watcher for most of the series) and Wesley Wyndam-Pryce.
Buffy, a high school student (now in college) in Sunnydale, Calif., discovers that she's the vampireslayer, and that her town sits on a "hellmouth" where demons are constantly trying to make an opening to escape from perdition.
The symbol is prominent in the show's opening, and Buffy wears a cross as a symbol of her role as a slayer.
In short, Buffy the VampireSlayer is a parable, a postmodern morality play in which Buffy is a Christ figure, her Scooby Gang is the church and the vampires and demons represent the variety of temptation and moral hazards we all encounter in life.