I'm going to qualify the walk-up statement.
There are two types of restaurants: Table Service, which are restaurants where you are seated, have a waiter, there might be a buffet, etc. etc. Your odds of walking up to most of these is low. Skipper Canteen, at the Magic Kingdom, takes same-day and walk-up reservations only. At Animal Kingdom, you might be able to get same-day for Rainforest Cafe or maybe Yak & Yeti's table service. Epcot's World Showcase has some less popular options that are a maybe, particularly at non-peak hours, such as Spice Road Table, Marrakesh and 9 Dragons.
Quick Service restaurants are a counter service, where you order at the counter and take your tray. Many of these are very good! Only one QS in all of Disney takes actual ADRs, which is to say the rest do not! At Magic Kingdom, Columbia Harbour House is delicious. You can get pizza at Pinocchio's Village Haus and overlook Small World. Cosmic Ray's is its own experience, ahem. At AK, Flame Tree BBQ is QS and fab. You can eat around the world at Epcot quick service stands. At HS... well, uh, there is food.

(QS there is weak.)
If you are on
Disney Dining Plan and have Table Service credits, make ADRs. At least look the night before and book. If you're paying all OOP and are okay with not having wait-staffed meals, character options or big ol' buffets, QS is always an option, and can often be a tasty one.