What does Jacob tell Bella about imprinting?

Jacob tells Bella how in the legends of the werewolves the old ones used to talk about imprinting, how a werewolf would look at a woman and realize she was the one he was supposed to be with. Jacob says that they had always thought these were just legends, but imprinting happened to Sam shortly after he began to transition into a werewolf when he saw his girlfriend's cousin, Emily