Some stores like to setup extras so that you can communicate to your customers what extras you offer with your bikes, like pedals, locks, helmets, insurance etc.
This means you can set them up before the customer arrives (and even charge extra money for them)
Part one is setting them up in your system.
Navigate to settings → extras and press the Add New button
Then you will see the detail page, which you just need to fill in:
When your customers make bookings the confirmation emails will include what extras have been chosen.