When you have Events (opportunities, signups etc.,) in which you would like to enable the community to submit events for you to review and approve to publish, or a way to create unique custom fields to track for every event and opportunity.
In order to activate this, navigate to "Customize", select "Events" as shown below.
Settings: allows you to control how events are viewed on your group page
Event Submissions: enables you to turn this on, select a specific date in which people can only submit events to and to customize the language further.
Administrative fields (a platinum option): enables you to customize fields to collect info about your events beyond the basic fields we have as a default.