It's possible you've already heard about it. It doesn't matter if it's called Recovery Phrase, Seed Phrase, or 24 Words. Your private keys are solely backed up by your 24-word recovery phrase.
Your PIN code is unique to your physical device, but your Recovery phrase is tied to your private key and hence to your funds. Even if you switch to an other device, it remains the same. And if it's detected, it'll allow them access to your money.
Your recovery phrase is a unique 24-word sequence created at random by your hardware wallet during setup.
This is the only time they are visible, and they are your sole financial safety net. There is no other backup because no third parties are engaged. You are the only one who has control over your finances.
If you forget your PIN number or lose your device, for example, your 24 words will allow you to recover access to your funds using your backup Ledger hardware wallet or any other wallet.
Finally, never, ever share or lose your 24 words. Maintain a safe and secure environment for them.
How? When your 24 words appear on the screen of your smartphone, carefully write them down (in the exact order and without any misspellings).
Then, after you've set up your hardware wallet, keep them secure. Every Ledger hardware wallet comes with a Recovery Sheet, which is a physical card made exclusively to hold your 24 words. Please study and carefully follow the recommended practises for protecting your recovery phrase and sheet. It is, once again, your obligation