When we load data, we need to load it in the order of dependencies; for example, before we insert Opportunities, we must first insert the Account records.
Another example is to load Opportunity Line Items. We must first load both the parent records of Opportunity as well as Price Book and its Price Book Entries.
Salesforce automatically manages the system fields, such as Created Date, Last Modified Date, Last Modified by, and Created by. Normally, we are unable to set or change these values for audit integrity purposes. However, an administrator can contact Salesforce Support to enable the Create Audit Fields feature for a limited time to allow these fields to be set on the create operation only.
To find more details about how to create Audit Fields, go to https://help.salesforce.com/apex/HTViewSolution?urlname=Considerations-before-having-Create-Audit-Fields-enabled&language=en_US.