pretty straightforward, you give openai api key, you receive a slightly improved workflow of copypasting your resume and job description into some other webpage, and then you get a cover letter back. (now you can copy and paste things without leaving this page, despite the pretense of this ux improvement being carried by how extension works anyway... yay)
the prompts are customizable, you can change the system prompt and the user prompt suffix. Honestly you can change a lot of stuff, for instance adding local infererence. I don't bother for now as I have models to train.