A dental dam is a square piece of stretchy latex or a similar material. It is placed over a woman's genitals when she receives oral sex. Oral sex is a sexual act where someone's mouth touches the other person's genitals. Genitals are the sexual or reproductive organs that are on the outside of the body (the testicles and penis of a guy, or the labia, clitoris, and vagina of a girl.) A dental dam helps prevent the spread of STDs (sexually transmitted diseases).
If a guy is receiving oral sex, he should wear a condom to prevent the spread of STDs.
You can buy dental dams online and at some drugstores. Some sexual health clinics have them available for free.