The Metadata API allows you to retrieve "data about data" for a given BambooHR account. You can determine what fields a customer is using. You can determine what options are available in a list. You can determine what users are able to access the system.
Metadata is important because BambooHR is highly configurable. A given customer's account may have drastically different settings from another customer. Using this API, you can determine what values the account supports.
Read only access to metadata does not require any special permissions. When write access to metadata is supported then the user will need the appropriate permissions to create new users, for example.