To begin, as needed by the supplementary question, a plot exposition is, essentially, the introduction to a story. In an exposition, the setting (time and place), characters, and background of the text is defined.
Chapters 1 and 2, of Mary Shelley's Frankenstein, offer the second exposition to the tale (the letters offering the first introduction to the tale).
Chapter one of the novel offers Victor's historical background. This includes information on his father, his mother, how they came to be married, how Victor came to be, and the adoption of Elizabeth. This chapter offers readers an understanding of Victor's family background and the setting of the novel.
Chapter two discusses the relationship between Victor and Elizabeth as they grew up, Victor's childhood, the addition of Henry to the Frankenstein family, and Victor's friend, Clerval. The second part of chapter two defines how Vicotr came to love and be enamored by science.
These chapters are important given they offer the background necessary for readers to understand Victor's love of family and science. Given that these two ideals clash later in the novel, their introduction is of the utmost importance.