Workflows that are not set to run immediately will be added to this queue. By default the queue processes 15 items every 5 minutes so actual run times will vary slightly. The batch size can be changed in settings.
When are events created
It’s important to note that queue events are only created based on the Run Actions workflow option. If this is set to anything other than Immediately a queued event will be created. This means that certain triggers like Abandoned Cart, Subscription Before Renewal and User Has Not Purchased For Period will not appear in the queue even though they are triggering based on a schedule.
A queued event can fails because its missing a data item. E.g. it supposed to email to a certain user but that user has been deleted. When this happens it remains in the Queue Events table for 30 days and is marked as failed. From the Queue Events table you can attempt to run the event again, it will not reattempt by itself.