According to MedlinePlus, the epiglottis is a flap of skin that prevents food and drink from going down the windpipe and into the lungs. It's located behind the tongue and in front of the voice box, or larynx.
MedlinePlus explains that the motion of swallowing causes the epiglottis to fold back and cover the voice box to keep substances from travelling in that direction. Once swallowing is completed, the epiglottis returns to its resting position.
WebMD reports that an infection of the epiglottis is potentially very serious. In "epiglottitis," the skin flap becomes inflamed and swollen. Sometimes, other tissues, such as the tongue and throat, are affected as well. Severe swelling completely blocks the airway. The condition is typically caused by infection or trauma.