This is where you can configure the settings for Account Types and Account Fields.
An Account Type is useful when you want to group your accounts and create more customised fields for each account type.
For more information you can read the following articles:
Creating and Editing Account Types
Account Fields are details shown for the Account Types. There are all possible kinds of Account Fields and options that can be enabled such as making them mandatory and choosing who can edit them.
Account Field articles:
Adding an Account Field
Account Group Fields
Adding a Group to an Account Group Field
Adding/Removing an Account from being a member of an Account Field Group