Wikipedia goes into great detail defining the term Tickler File and how it fits into the GTD philosophy. Simply put a Tickler File is a method of reminding oneself about tasks that need to be accomplished at some future date. A good discussion of the method is described here.
A tickler file is a means of insuring that future appointments or tasks don’t slip through the cracks. The whole concept of 43 Folders makes this workable. In a 43F system you have 12 folders labeled Jan, Feb, … Dec. You have 31 folders numbered consecutively 1 through 31.
On November1, you open the November folder and load the appropriate daily (1 through 31) folders with the tasks or appointments that are due that day. As new items come in, file them in the folder for the month they are due.
The discipline required to make this happen is minor, and it works!