She is the 29th victim of Jason Voorhees.
Mrs. Jarvis was single mother, living with her daughter and son near Crystal Lake. She had been married since the early 1950s/late 1940s. She was a good mother who always made sure Tommy and Trish were alright. Mrs. Jarvis was off-screen killed by Jason Voorhees. His mother's death haunted Tommy for many years.
Mrs. Jarvis' fate and death would have been more further explained in a deleted scene that had been cut from the film. An alternate ending to the film, included in the 2009 Deluxe Edition DVD, shows a dream sequence where Trish and Tommy wake up the next morning after killing Jason to the sound of police sirens. Trish sends Tommy to summon the police who have arrived next door. At that point she notices water dripping from the ceiling and goes to investigate. She enters the upstairs bathroom, and finds the body of her mother floating in a tub full of bloody water.
Trish lifts her mother out of the tub, prompting Mrs. Jarvis' eyes to open, revealing them to be solid white and devoid of irises. Jason suddenly appears from behind the bathroom door and prepares to attack Trish. Trish then suddenly wakes up in the hospital in a scene reminiscent of the ending of the first movie. In his commentary, the director says this scene was cut because it interfered with the idea that this would be the final film.