Frankenstein is a novel written by May Shelley, not a play. Therefore, it is divided up into chapters and not Acts. I will assume that what you are looking for is the purpose to the opening letters and chapter one.
The opening letters of the novel introduce the character of Walton. Walton is an explorer who searches for a Northern sea passage to the other side of the world. He hopes to unlock the mysteries of the earth by exploring the unknown regions of the Arctic. This sets up the parallelism between characters once we meet and hear Victor's story. Walton also serves to introduce, not only the theme of exploration, but the main character Victor whom he finds floating on a slab of ice. This sets up the frame narration through which we hear the story of Victor as "a cautionary tale".
Chapter one gives us background to Victor. The main purpose to this is to show his family and childhood. It sets up many different character foils as we see the overwhelming generosity of these characters and the later selfishness of Victor. We get the information on his father and mother, how they met and the kind and generous ways of the family. This all contrasts Victors blind determination later.