What does it mean to add an account to my watchlist?

In your Lockout Folder, underneath each user card, there's an option to "Add to watchlist". When you add someone to your watchlist, they'll show up in the tab of your Lockout Folder called "Watchlist", and any new mentions from them will show up in there. You may want to keep the user muted, for example, but still keep an eye on them. This way, you can clear them out of your "Muted / Unreviewed" tab but still see updates from them. When there are new mentions from accounts on your watchlist, you will also get an on-site notification, unless you disable the notification setting.