The Soothe Bell held item has been around for quite some time in the mainline Pokemon series. The item is meant to be held by a trainer's Pokemon and what it does is calm the holder with its sounds, increasing its friendship/happiness levels. A Pokemon's friendship level really doesn't impact how it performs in battles. Still, some Pokemon can only evolve into their next form if they reach a certain amount of happiness/friendship.
In Pokemon Brilliant Diamond and Pokemon Shining Pearl, the
Read more on gamerant.com