Cloud bread is a low-carb, gluten-free bread substitute made by whipping egg whites into a meringue-like foam, then folding in the yolks (usually with cream cheese and a little sweetener) and baking the mixture into light, airy rounds. It has virtually no flour and very few carbohydrates, which is why it's popular with people following keto or other low-carb diets.
The idea goes back further than its recent popularity: a similar egg-and-cottage-cheese roll appeared in Atkins diet cookbooks in the 1970s. It resurfaced online in the low-carb community around 2008 as "Oopsie Rolls" - by one account, cream cheese was swapped in for cottage cheese by accident - and has since been rebranded and popularized as "cloud bread."
In the kitchen, cloud bread stands in for regular bread in sandwiches, burgers, and open-faced toasts, and its neutral flavor takes well to savory additions like garlic and herbs, or a touch of extra sweetener for a dessert version. It can also be made with almond or coconut flour blended in for more structure.