just need a simple explanation to understand!
Sets are your private watchlist of users on CodeChef. You can create multiple sets, and add multiple users to each set. As of now, you can only see the overall ratings and country of each of the users in your sets.
To add anyone to your set, you need to search them by username. You can narrow down your search by applying various filters such as Ratings, Country, Institution name, and Language.
You can also apply a filter in the ranklist of any contest by your set(s).
Read more about it in detail here.
Please feel free to let me know if you have any more questions.