A dental dam is a smallish, square piece of latex that is used as a barrier during oral sex to help prevent the spread of STDs (sexually transmitted diseases).
Dental dams get their name from their original purpose: They are used by dentists during dental procedures to block off the area being worked on.
The latex material most dental dams are made of helps protect against infection because it blocks body fluids. That's why people started using dental dams as a way to practice safe oral sex. For people with latex allergies, non-latex versions are available as well.
You can buy dental dams at some drugstores as well as clinics like Planned Parenthood.