Walking is essentially when you book a room in advance and then continue to modify the dates until you get to the date you want.
Take today for example, it's currently 4/27/23 meaning I can book as far out as 3/27/24 for a stay that starts on the 27th of March and ends on 4/3/24. Say if I wanted to guarantee a December stay at CCV in a studio and I had a February use year. I can book a studio right now and then keep modifying the dates every couple days allllllll the way until it's currently January 1st 2024 at which point the 11 month window would be open for a visit to stay from 12/1 to 12/8.
This is obviously an extreme example but it can be done nonetheless and does happen especially with studios at Crescent Lake resorts during Food and Wine season and also special holidays say like theme park view studio at Bay Lake Tower on July 4th/NYE. This also happens for CCV during early December. Usually people only walk a few months though.
In general, it's really unnecessary and most would say rude to people who actually need rooms at specific times because it prevents them from booking a reservation they may actually need which is why it's frowned upon.