Papers searches databases like PubMed and Google Scholar to intelligently find papers that might be of interest to you based on the papers you already have in your library, as well as your recent activity.
You can select the time frame from which recommendations are pulled, change the sorting order, as well as choose which recommendations you're getting based on the library and/or list you select. You can also select whether to receive them for articles that are open access or not, or either of the two.
If you do not see any recommendations in Papers, you may not have enough articles in your library. Add more articles by drag-and-dropping PDFs directly into your library, or clicking the “Add” button at the top of the library window.