Mr. B started out making fun of Pamela for overreacting and getting mad at her for telling on him.
Mrs. Jervis, all nervous because it's probably her job on the line too, tried to downplay the seriousness of the master's behavior, which—of course—made Pamela cry.
Oh, sure, Mr. B said, Pamela is pretty and all—but he's just playin' around.
This suggestion that his harassment was a "jest" (19.13) got Pamela angry all over again, and then Mrs. Jervis got mad at her.
The meeting ended with Mr. B resolving to fire Pamela and send her back to her parents.
Thus, she is writing now to let her parents know to expect her as soon as she can wrap up any remaining tasks.
Woohoo! Book's over; time to take a celebratory nap.