When you first setup the device you create a 24 word passphrase. That phrase essentially becomes your new private key. So you'll want to write it down on a piece of paper (or two) and put them in a fireproof safe or bank deposit box.

The Ledger then uses those 24 words to setup your new private keys on the device itself. If anything were to happen to the device, you can use those same 24 words to restore on a new ledger device and it will be the same as your old one. You can also take those 24 words and use them on web wallets to recover your private keys and funds if for some reason Ledger went out of business.

Once your Ledger is setup you then set a 4 or 8 number pin on the device (I use an 8 number pin) and everytime you plug the device in you just put your pin in and it unlocks the wallet. You don't ever have to use the 24 words unless you need to restore the device.

The device protects against brute force attacks against your pin number by only allowing an incorrect pin to be entered 3 times. If you put the wrong pin in a fourth time, the device will reset itself and delete everything on it. You would then end up having to restore the device using your 24 word phrase.

When you get your wallet I recommend going through that process without any funds on the wallet just so you can see how it works and trust that you can restore if you ever needed to.