The Friends List 
is a list where you can see your friends.
Friends are users that you can add via the Friends List .
How to add a Friend
To add a friend, you must press the "ADD FRIEND" button. Then, enter the friend's username. Then, get your friend to do the same thing, but with your username. Note that the friend will not get a notification on this, so you must communicate through the chat or other means. Then, you have succesfully added your friend.
What adding a Friend does
After you add a friend, you will now see all their skill's XP and level, including the individual Combat Skills and Invention, even if they haven't unlocked it yet, and also their trophies.
You will also see their current playtime, if they are online, their mode that they have chosen to play on, the amount of Coins, Energy and Stardust they have, as well as a button to see their profile.
The best player in your friends list will get the LEADER status on it, this does absolutely nothing.