I didn't know about this until the first time I flew with DD! It basically means that you don't have to check your stroller with your luggage when you get your boarding passes and stuff. You can take your stroller with you to the gate where you'll wait for your plane. When it's time to board, you'll tell the flight attendant who takes your ticket that you want to gate-check your stroller, and he or she will give you a tag (similar to what you'll put on your luggage) to put on the stroller. Then, when you walk down the ramp toward the plane, you'll leave your stroller at the bottom of the ramp. The flight attendants will stow it somewhere, and then when you land, they will get it out for you (you might have to wait a minute, but you'll see other people waiting at the door of the plane after exiting, looking for the stuff they gate-checked).
This way, you can use your stroller in the airport, as I mentioned, which might make your walk to and from your plane much more efficient. This (to me) is the best way to do it if your child is young enough that he/she will ride in a stroller... I've read on here that people bring strollers to WDW for their 6- or 7-year-olds because of the incredible amount of walking that going through the parks requires. If that's the case, it's probably easiest just to check your stroller with the luggage, but if you can get your child to ride in the stroller at the airport, I think gate-checking it is best. Also, you eliminate the possibility of it getting lost or not arriving on the same flight as you do (that can be important if you're taking a car seat, too, but that may not be the case since you're going to WDW).
That was a long-winded response! I hope it helped
