The Watchlist is a list of TWILs unique to you. You can choose to place a TWIL in your watchlist for many reasons - its a TWIL you want to refer to often, or its a TWIL you haven't gotten to yet. To place a TWIL on your watchlist, there are 3 vertical dots on the TWIL screen. If you click on them you will see Add to Watch list. Click again to add.
TWILs on your watchlist appear on the home page under My Watch List.
You can also remove a TWIL from your watch list. Click on the 3 vertical dots on a TWIL that is on your watch list. You will see Remove from Watchlist. Clicking on Remove will take it off your watchlist.