In a professional kitchen, a walk-in refrigerator is a refrigerator that you can physically walk in to. This is different from a "reach-in" which you can open the door of and reach into similar to what you might find in a home. There are also walk in freezers and sometimes walk-in refrigerators with walk in freezers in the back.
Walk in referigerators can be all encompassing providing every cold storage need for a restaurant or it may be highly specialized such as a referigerator that is temperature optimized to hold mushrooms or other delicate produce such as tomatoes. It isnt uncommon to see refrigerators designated as "protein" walkins "vegetable" walkins dairy walkins and service walkins. The latter intended to hold prepared components ready for service and located very near the line cooks.
Walk in refrigerators generally contain shelves and speed racks designed to hold hotel pans, deli containers, and sheet trays of food. Walk-ins are also designed in compliance with department of health regulations and likely contain multiple thermometers, an adjustable temperature overhead mounted cooling unit, and everything in the refrigerator is required to be labeled and dated in accordance with local health regulations. There are also rules governing how proteins are stored with cooked foods being stored above cold foods, and then top to bottom fish, beef, pork, and poultry to minimize the chances for cross contamination.
The term "walk-in" can also refer to a guest or set of guests who came in to dine without a reservation. Eg "we just had a 4 top walk in right before close."