To use your Chain Breaker, merge it with a chained item to break the chain on the item.
When can I use the Chain Breaker?
Chain Breaker can be used when you reach its highest (max) level.
At the beginning of the game, you have chained items on your board. You can use your Chain Breaker to break the chains on these items and set them free.
When you tap on a Crate and spawn chained items, you can use your Chain Breaker. Try to drag it on a chained item and merge them to break the chain on the item.