The address book is a list of addresses, which you interact with often (such as sending funds, for example) or would just like to save for future use. Here is a step-by-step explanation on how to add and remove addresses from your address book.
Step 1: Go to the Transfer tab in the left side menu and click on Add Address in the Address Book section on the right side of your screen.
Step 2: Insert the name under which you want to file the address, as well as the address hash. You can also add a domain name from Unstoppable Domains. Click on Confirm at the bottom of the window to add the address.
Step 3: You will be able to see the newly added address in your Address Book. If you wish to delete it, you can simply click on the trash bin icon on the right side of the address.