It's the blanket name for their security architecture. The thing that makes sure your kernel is blessed, tries teo tell if you're rooted, then sets a fuse flag if anything is off. It also provides a secure, encrypted profile for your phone that bifurcates apps, data, blocks screenshots. The data from the flag is available to apps to tell that your phone is potentially insecure. For the most part, they only block Samsung banking/pay apps and make your secure partition inaccessible.

My next phone will be something degoogled. hopefully something linux.

I've already wiped an old disconnected android phone for use with my drone/cameras that require a mobile device.

According to the linked article it prevents the use of Samsung Pay and access to the Secure Folder (an extra layer of security you can enable that requires a second PIN to be input before you can access certain apps and files). This seems pretty reasonable, the goal is clearly to prevent access to especially sensitive data if someone has stolen the phone.

For me I found out when I wanted them to fix something and they refused to honour the warranty because of the blown fuse.

As far as I know, this is illegal, btw. They have to prove that the error you are reporting is caused by user action. If your battery craps out, they can't blame it on you rooting your phone.