Just to make life more interesting for you. You should be using the 24 seed words plus the secret. Partly for plausible deniability of you are forced to unlock the ledger. But more importantly, you need to store the 24 seeds somewhere securely. But if anyone gets hold of them, then they own your crypto.

With the 24 plus one, you don't store the two parts together.

So, ledger have added another layer of security with the extra seed that is secret. It will generate a new set of addresses and wallets, this you have two sets. One for the 24 seeds that can work as a short term place to keep stuff. And the 24 plus secret that can act as a storage for larger amounts.

EDIT: I see from the comments that there are a lot of new people to the crypto ecosystem. Please, don't buy hardware wallets from anyone other than the manufacturer. The hardware wallet will generate the seed words when you set it up, it will never never never be pre set up, it will never have the words on a piece of scratch here paper. Please be safe, ask people that know what is going on to help you.