A dental dam is a small, square piece of latex or a similar material. The material is thin, but tight enough to block liquids and germs from getting through.
Dentists use dental dams to keep a part of the mouth dry or free of germs while they work on it. For example, a dentist might put a dental dam around a tooth to keep saliva (spit) from getting on it during a dental procedure. Some dentists use dental dams to stop small pieces of tooth, water, or other liquids from going down a person's throat. After the dentist has finished working, he or she takes out the dental dam.
Because dental dams are made from a material that's similar to condoms, lots of people use them during oral sex to block body fluids and help prevent the spread of STDs (sexually transmitted diseases). You can buy dental dams at some drugstores as well as clinics like Planned Parenthood.