Parallel Mode

If checked (Step Properties window), all the Operations in the Step will be run at the time the Step is run.  In other words, the Operations in the Step are run concurrently.  The Step is complete when all of the Operations have completed.  When Parallel mode is not checked (the default) the Operations in the Step are run the order they are listed in the "Operations in this Step" list.  When the first Operation completes the second Operation is started and so on.

Example use of Parallel mode:

You may have a Job that is dependent on five Operations.  The Job should not run until all five of the Operations have completed.  An object-to-object Dependency can be created for the Job being dependent on the completion of the Step.  All five Operations are started at the same time with the Step.  When they have all completed the Step is complete and the Job can now start.