To access your vault an attacker will need both your master password and the secret key. These are effectively combined to generate your keys for decryption.
This protects against an attacker gaining access to 1Password servers. They can’t control whether you chose an awful password or not. So to protect them the secret key adds a ton of protection for those with weak, reused, or compromised passwords. Even in those cases an attacker needs to guess the secret key alongside the awful password. Using both the secret key and a strong master password is basically the equivalent of making a vault incredibly secure and, uncrackable using todays technology.
This does not protect against local compromise of a device of yours though, as the Secret Key is stored on device and is accessible. This prevents you from having to type it every time.