On Polkadot-JS you will see an Accounts Page as well as an Address Book

What is the difference between them? 

1. Accounts Page

In order to send funds out of an address, you need to control it with your keys. Any address you have under "Accounts" is controlled by you and you can therefore send funds out or perform other actions with it. 

Addresses that show up on your accounts page have either been created or restored by you in the polkadot{js} browser extension or in one of the ways highlighted in the image below. In both cases, you control those addresses (because you own the private keys to them). 

2. Address Book

Addresses that show up in your address book are contacts. They do not require private keys to be added there, allowing you to add any address here as a contact. 

Here you can, for example, add your DOT addresses at exchanges that you'd like to monitor or regularly send funds to, or your DOT address of your Ledger. Your address book is a passive "watch tool", plus it makes it easier to send funds to your contact addresses. 

You cannot send funds out of an address of your address book, since it's just a contact address. (Just like you cannot send an email from a friend's email address that you have in your contact list.)

How to move an address from the address book to the accounts page 

The only way to do this is if you own the private keys of the contact address (in the form of a JSON file or mnemonic seed). In that case you could use those keys to restore access to those addresses on the accounts page. Here is how to do that.

In the case of your Ledger, you would simply connect your Ledger device with the "Add Ledger" button.