Mainly for foreplay and giving head.
It is used as protection against contracting STD's htrough the mouth.
 
How To Use A Dental Dam As A Barrier For Oral Sex
Dental dams are latex or polyurethane sheets used between the mouth and vagina or anus during oral sex. Ready-to-use dental dams can be purchased online.
Dental Dam Dos and Don’ts
	
DO use a new latex or polyurethane dental dam every time you have oral sex.
	
DO read the package and check the expiration date.
	
DO make sure there are no tears or defects.
	
DO put on before starting oral sex and keep it on until finished.
	
DO use water-based or silicone-based lubricant to prevent breakage.
	
DO store dental dams in a cool, dry place.
 
	
DON’T reuse a dental dam.
	
DON’T stretch a dental dam, as this can cause it to tear.
	
DON’T use nonoxynol-9 (a spermicide), which can cause irritation.
	
DON’T use oil-based products like baby oil, lotion, petroleum jelly, or cooking oil because they will cause the dental dam to break.
	
DON’T flush dental dams down the toilet as they may clog it.