MCB (Miniature Circuit Breaker)
They are used to replace fuses that protect individual pieces of electrical equipment and the circuits which supply them with power.
In the case of tripping because of a current overload, MCBs are very easy to reset, which is much simpler to do that having to replace blown fuses. MCBs also trip to stop the current from flowing much faster than fuses can operate.
However MCBs are not better than fuses in all respects: MCBs are much more expensive than fuses and they take up more space, which is why fuses are still used for some applications.