To what is Mary Shelley referring when she says, "I have not seen these stories since then, but their incidents are as fresh in mind as if I had read them yesterday" in Frankenstein?
Mary Shelley and her husband, poet Percy Bysshe Shelley, went to stay with their friend and poet, Lord Byron, in Geneva, Switzerland, in the summer of 1818. In the "Introduction" to Frankenstein, she talks about this visit and how it was the cause of her writing this novel. The line to which you refer in your question comes from this introduction.
She says it was a particularly "wet, ungenial summer, and incessant rain often confined us for days to the house." They spent some time reading some books containing ghost stories translated from German into French, and she recalls several of the stories.
One was titled "History of the Inconstant Lover," about a man who gets married but, as he is embracing his new bride he discovers he is holding the ghost of the woman he once "deserted." Another related the story of a man, "the sinful founder of his race," who had to kiss each of his youngest sons "just when they reached the age of promise." The man was a ponderous ghost who sorrowfully kissed each son who, "from that hour withered like flowers snapt upon the stalk."
After she recalls these stories, Shelley says,
I have not seen these stories since then; but their incidents are as fresh in my mind as if I had read them yesterday.
In this line, Shelley says that, though she has not seen for many years the ghost stories they read that dreary summer, she remembers them as vividly as if she had "read them yesterday."