Bird's nest soup is a dish that is literally what it sounds like: a soup made of bird's nests. However, this soup is made of a specific type of bird nest that is formed through an extrusion of the bird's saliva rather than an assemblage of twigs and other materials.
The bird in question is the cave-dwelling swiftlet, which is a species that is endemic to Southeast Asia. Bird's nest soup is prized not only for its flavor but also for some supposed medicinal properties, which include an increase in virility, digestive performance and immunity. This is mostly a novelty, though, and it's not something many people eat regularly.