"Flatten" can mean many things. With Acrobat flatten means converting objects like annotations or form fields from their object layer to the content layer and make them part of the content of the PDF. One can still use Acrobat to edit the items using the correct tool. Once "flattened" with Acrobat one can not restore the flatten objects back to their original object type.
Unlike Redaction, which leaves an original PDF and makes a redacted copy, "flatting" does not make a copy of the PDF. In the future you might want to make a backup copy before performing this action.
For others, "flatten" means making the PDF uneditable or harder to edit.