Lists are a great way to view posts from a subset of users you follow. Mastodon only lets you add people you follow to a list. Also, when you add users to a list, you won’t see any of their posts prior to the time you added them.
You can add members to a list from the list tab, or go to a user you follow’s profile and choose Add/Remove from Lists in their profile options. At this point you can optionally choose to filter them out of your home timeline. This is our way of letting you add people to a list and not see their posts duplicated in your timeline and a list.
You can read posts from a list from the list tab, or by tapping on the Home title button in your timeline and selecting one of your lists.