Usually each resource can be provisioned independently. E.g. LDAP account and database account in application X are not in any way related to each other. MidPoint assumes that they are independent and may take advantage of that e.g. by provisioning these two accounts in parallel.
However, there are some cases in which the accounts are not independent. E.g. an operating system account must exist before an application account can be created. Or one of the resources generates attribute value (e.g. e-mail address) that is required to create another account. The That is the case of dependent accounts: one account must be provisioned before the other.
MidPoint by default assumes that accounts are independent. If the accounts are dependent, then such a dependency must be specified in the resource configuration: