A food cover or food umbrella is used to protect finished food or ingredients from being attacked by flies, wasps and other insects and animals. A food cover or food bowl cover is an elasticated piece of fabric that can be stretched over a bowl or deep dish to preserve food, this isn't an air-tight seal just something to protect the food, ideal for bbqs and parties.
A food umbrella is much the same as a food cover except that it creates a sort of 'tent' around a plate of food, this way bigger, flatter food vessels can be protected. These roll or fold relatively flat and then spring out when they are needed. Again their purpose is to protect food from insects and animals rather than to preserve food for any duration.