A standing buffet you stand and talk to whoever is in your party or mill around and talk to other guests while you eat what you have put on your plate (usually finger foods) as versus serving ones self then sitting at a table to eat.
A standing buffet wedding reception is a situation where we have not provided a seat for every single guest. They do not have a particular place to sit. The seating might be small cocktail tables, or stand-up cocktail tables without barstools, or chairs. The guests would typically go to the buffet. It would be a smaller plate. They would get some amount of food, sit for a while and eat, perhaps go back to the buffet, and get something else. There would be a lot more movement in the room, as opposed to just sitting down and eating a meal straight through.