An Item Bank is a repository of test questions and any information pertaining to those items. In education, they are typically defined by subject (e.g., Math, Biology) and, in other situations, are often defined by a credentialing/assessment program or job family (e.g., Entry Level Certification, Advanced Certification).
Authoring items is often the first step in test development. Items reside in item banks, so you have to create item banks before you can create items. For example, just like on your PC, you create a folder and then put files in the folder.
Users can be assigned rights to view and edit specific item banks while Administrator (admin) users will see all banks. For example, Math teachers can be limited to the Math item bank.