Comment by buran77

The HSM is more generic than that - a Hardware Security Module. It's just a hardware (usually, software... Hardware security modules exist...) device that securely stores your secret cryptographic material, like certificate private keys. The devices are exceptionally hardened both physically and the running software. In theory any attempts to attack them (physically open, or even turn them upside down to investigate them, or leave them unpowered for longer than some hours, attempt too many wrong passwords, etc.) results in the permanent deletion of all the cryptographic material inside. These can be server sized, or pocket sized, the concept is the same.

Their point is to ensure the private keys cannot be extracted, not even by the owner. So when you need to sign that firmware update, or log into a system, or decrypt something, you don't use a certificate (private key) file lying around that someone can just copy, you have the HSM safely handling that for you without the key ever leaving the HSM.

You can already guess the point of a cluster now. With only one HSM there's a real risk that a maintenance activity, malfunction, accident, or malicious act will lead to temporary unavailability or permanently losing all the keys. So you have many more HSMs duplicating the functionality and keys. So by design there must be a way to extract a copy and sync it to the other HSMs in the cluster. But again, these are exceptionally hardened HW and SW so this in incomparably more secure than any other transfer mechanism you'd run into day to day.