Jessica Lockhart is a fictional character on Days of our Lives, played by the ficitional actor Cecilia Monroe and portrayed by Susan Sarandon. She's only seen once, when she dies and Drake Ramoray, played by Joey, gets her brain, so he can awake from his coma, a procedure Ross takes issue with on medical grounds.
She was famous for throwing drinks at people and slapping them. Her daughter, named Dina, was seeing a man named Frederick, which she did not approve. She was thrown off a horse and onto an electric fence, an accident set up by Frederick and Dina. After handing over the role of Jessica Lockhart to Joey, Cecilia Munroe slept with Joey causing minor hysteria for Rachel and Monica, who both are big fans of Days.
Later scenes exploring Joey's role in the show are unclear on the state of his current character, with comments suggesting that Joey is either Jessica in Drake Ramoray's body or Drake with access to Jessica's memories.