If your site has a password-protected area and a logged in user goes to a donation page, their First Name, Last Name, and Email Address will be autopopulated in the donation form. If you have Salesforce integrations enabled, their Organization will also be autopopulated. This behavior is automatic and does not need to be enabled.
Should you wish, you can choose to disable contact field editing for logged in users so that the autopopulated information cannot be changed for the sake of data integrity.