The first time you use the app you will need to logon.
Tap the 'Settings' button on the bottom right of the screen.
Tap 'Add Library Card'.
On the new screen, type in the borrower number starting B00...
You will find it on the back of your Library card
Add your PIN number.
The navigation bar at the bottom of the screen can also tell you what you have on loan, anything reserved and what you have borrowed in the past.
You can even set alerts to remind you when you loan is soon to be overdue.