This is the mucus membrane.
Mucus membranes are found lining the body canals and cavities that lead to the outside namely the respiratory tract and digestive tract as well as the urogenital tract.
Mucous membranes secrete a sticky substance called mucus which is made of a monopoly saccharide called mucin.
The mucus traps pathogens and other particulate matter, preventing them from getting to deeper tissues. Once trapped, these unwanted foreign intruders are expelled outside of the body with the mucus, or swallowed into the stomach where they are destroyed by the strong acid there.