Dental dams are an STI prevention barrier used during oral sex for vulva owners. It is a rectangular flexible piece of latex that can be placed on top of the vulva before oral sex. Dental dams are a one time use products like condoms.
Sources:
https://www.verywellhealth.com/what-are-dental-dams-906812
https://www.webmd.com/hiv-aids/dental-dam-how-to-use
https://www.cdc.gov/condomeffectiveness/Dental-dam-use.html
https://www.healthline.com/health/healthy-sex/dental-dam#_noHeaderPrefixedContent