It's either really general or really specific. I'll stab at both:
Generally, a touring plan is simply your planned daily itinerary for a day at one of the Parks. You know .... 1. Arrive 8:30 2. Ride Toy Story Mania 3. Ride Rockin' Roller Coaster ... that kind of thing
Specifically the term was pretty much popularized by the Unofficial Guides books and they have a HUGE variety of VERY specific suggested touring plans for each Park based on a lot of different factors. There are others who put out touring plans, although the website touringplans.com is associated with the Unofficial Guide to WDW. These are a very helpful resource if you are serious about optimizing your time doing something other than standing in lines.