When creating a workflow rule, there are 3 options for WHEN your criteria will be evaluated by the rule.
Evaluate the rule when a record is: Created
If the evaluation is chosen as 'when a record is created' ONLY, the workflow cannot fire on records that have already been created; only new records that meet the specified criteria when they are created. 
Evaluate the rule when a record is: Created, and every time it's edited
If the evaluation is chosen as 'when a record is created, and every time it's edited', the workflow will evaluate the record every time it is edited and saved. If the record meets the criteria, the workflow rule will fire. Since this option evaluates and can fire so frequently, you are not able to add time-dependent actions. The only actions available to add to this workflow rule will fire instantly when the record is saved.
Evaluate the rule when a record is: Created, and any time it's edited to subsequently meet criteria (recommended for most workflow rules)
If the evaluation is chosen as 'when a record is created, and any time it's edited to subsequently meet criteria', the workflow will evaluate the record every time the record is edited and saved. The rule will fire 1. when the record goes from NOT meeting the criteria TO meeting the criteria OR 2. when the record goes from meeting the criteria, to NOT meeting the criteria, then edited and saved to meet the criteria.
Ex. If a workflow rule is set to fire when an Appointment has a Start Time, when the Appointment is edited and saved with a Start Time, the workflow rule will fire. If you need the workflow to fire AGAIN, you will need to edit the record and REMOVE the Start Time, then save. Then re-edit the record, and enter the new Start Time, and save. The record has gone from meeting the criteria, to NOT meeting the criteria, then back to meeting the criteria and will fire.
