In the Azure Container Registry blade in the Azure portal, there is a "Lock" section on the left hand side and I can't find it anywhere in the ACR documentation.
What is it?
Its a general Azure (not specific to ACR) feature to prevent accidental deletion (or modification) of a resource. You can create a lock that would prevent users from deleting or modifying the resource while the lock is there (even if they have the permissions to delete\modify the resource).
https://learn.microsoft.com/en-us/azure/azure-resource-manager/resource-group-lock-resources