A pending user is someone who requested access to your library via the Account Registration screen. Admins can active these account requests by approving them and assigning them a proper permission.
Go to Users & Permissions, and click on Pending Users.
Find the user and assign them a permission level. Once you have done this they will be automatically sent an email detailing their account information. They will then be listed in the Active User list.
You may also Decline a pending user if you wish. They will receive no further contact from you in that case.