Charles gave a good explanation, but I'd add that to fully "walk" a reservation, you would make the initial reservation as indicated on January 18 to book Dec 18-25.
Then you would call back the next day (19th) and book one more day at the end and drop the first day of the reservation.
You would continue this every day until you get to the point where you have booked your originally planned reservation.
It is a way to provide an advantage versus waiting until January 24 to book the entire trip. When Disney changed the reservations from 11 months before the check-out date to the check-in date, this practice replaced the previous "day by day" booking strategy that some employed.
I'm not condoning or condemning the practice, just explaining it.