In the parlance of GettingThingsDone, a TicklerFile consists of a series of 43 filing folders: 12 monthly and 31 daily folders. Items are filed in a folder corresponding to when the item will need to be handled, thus reminding or "tickling" the user about it. These items might be action reminders, bills, receipts, reports, letters, or any number of other things.
A certain day's (today's) "to-do" items can be found in a single folder and can be taken into processing in the morning, for example. Some items can be pushed back to a later day's folder or archived and the others processed.
http://wiki.43folders.com/index.php/Tickler_file