Before watching the movie:
Similar to how the Batman franchise spun off Catwoman, after three Christopher Reeve movies, the Superman franchise spun off Supergirl. However, while Catwoman was basically made because the next proper Batman movie was in development hell, my understanding is that Supergirl was a pure spinoff attempt riding on the success of Superman. While Catwoman is generally considered awful, it was followed by the acclaimed Dark Knight Trilogy, and I get the sense Supergirl is considered okay but forgettable, and it was followed by Superman IV.
I know exactly what to expect from the plot because this is the introductory movie of a superhero, which has been required to be the rote origin story for around 40 years, if not longer. As for a villain to slot in, I’m not sure what to expect, since I don’t know of Supergirl having any villains that are “hers”. I’m really not that familiar with her at all besides the basics of her relationship with Superman, so maybe I won’t have lore bogging down my enjoyment.