A jian (sword) breaker is a special weapon that is used to break, destroy or disarm an opponent's sword during a fight. It looks like a small, curved sword with a gap in its middle. The gap is there to help it trap the enemy's weapon.
Imagine if you were skipping rope and someone else tried to jump in the rope at the same time. Instead of just holding onto the rope, you could trap their foot and cause them to fall. That's how a jian breaker works. When your enemy swings their sword at you, you can catch it in the gap of your jian breaker and then quickly twist, causing the enemy's sword to break or bend.
This gives you an advantage because if your opponent's sword is damaged, they won't be able to use it as easily to defend themselves or attack you. A jian breaker can be a really useful tool if you know how to use it properly, kind of like how a lock-picker can use their skills to open locked doors.