If you like someone's closet on the app, then you can start following them just by tapping on the follow button on their profile. Following helps you to stay updated to their new listings and to keep a track of their new items. If you wish to follow a particular user, go to their user page by tapping the profile picture on either their listings or comments. Then tap to Follow.
You can check out the updates from people you follow in "Following" feed on home page.
You can check out all the people you follow in "People You Follow" in the side menu.