The Tickler folder is a place to keep those non-actionable items that you need to review at a later date. For example, if you get an email from a friend about a Bruce Springsteen concert in August 2013, you could create a tickler entry from that email and revisit it when tickets go on sale.
By doing this you can reach Inbox Zero, and you'll have the peace of mind that you won't drop the ball on anything.
You create a Tickler just like you would an Action.
Fill out the important information, such as Short Description.
Set the Follow-up date to the time you'd like to be reminded.
You will receive a notification at that time.