In the Automation designer, there is a setting to allow re-entry into the Automation if a contact meets the criteria to enter more than once. Use the toggle to set Yes/No and set # of days to define timeline between exit and re-entry.
You may want contacts to re-enter an Automation when-
1) A contact has re-subscribed, send them through a general welcome series again.
2) A returning customer makes a purchase, send them through a post-sale series again.
You may not want contacts to re-enter and Automation when-
1) An account setup series where steps may have been completed in the past, like setting a login password.
2) A trade-show follow-up series that would be specific to the event.
Contacts that may meet criteria to re-enter before the configured # of days will be skipped. Contacts are not collected and held until the configured # of days.