From the MP overview thread (excellent resource! read it, if you haven't already) linked above (in HydroGuy's post):
"Stacking" FPs:
When people reference "stacking" FPs, they're talking about booking FPs so that you have a bunch ready to use at one time. This is especially nice if you're taking an afternoon break and want to have a bunch of FPs ready when you come back (just keep booking a new one every 90 minutes). Some examples of how this can work:
Example A - stacking for the evening:
Mid-morning, you book RSR for a 7:00-8:00pm return time. Throughout the day, you keep an eye on other DCA rides and book them when they get to evening returns - say, grab GOTG with a 6:30-7:30 return time, and later grab Incredicoaster with 7:15-8:15. Now, when you come back to DCA, all three of those are waiting for you.
Example B - stacking by location:
First thing in the morning, you book Space Mountain with an 8:45-9:45am return time. At 8:45, you book Star Tours with a 9:15-10:15 return time. At 9:15, you book Buzz Lightyear with a 9:20-10:20 return time. Now, head to Tomorrowland and ride all three. (Note: In this case, I'd actually ride Buzz first so you can grab your next FP immediately - keep in mind that your Space FP will scan until 10am, giving you plenty of time to hit Buzz on your way there.)