The abort parameter is a Boolean value.  If abort is true, then all changes associated with the current edit will be cancelled.  Also, if the current operation is a standard Relate edit, then the user will be returned to the edit screen and be given the opportunity to revise their changes and try again. 

Things which will be cancelled include changes made by the user, changes made by this and other formulas, and email and intramail messages sent by formulas.  In technical terms, sending an abort causes a database rollback to occur for the current transaction.

If an abort is sent during the execution of a scheduled formula (either by the scheduled formula itself or by another formula triggered by the scheduled formula), only changes to the current record will be cancelled.  Changes to other records will still be completed and stored.

If an abort is sent during an import, the entire import will fail and the message-string will be displayed as an import error.