A clit shield is a body piercing

Body piercing

Body piercing, a form of body modification, is the practice of puncturing or cutting a part of the human body, creating an opening in which jewelry may be worn. The word piercing can refer to the act or practice of body piercing, or to an opening in the body created by this act or practice...

 used as a chastity

Chastity

Chastity refers to the sexual behavior of a man or woman acceptable to the moral standards and guidelines of a culture, civilization, or religion....

 device which covers the clitoris

Clitoris

The clitoris is a sexual organ that is present only in female mammals. In humans, the visible button-like portion is located near the anterior junction of the labia minora, above the opening of the urethra and vagina. Unlike the penis, which is homologous to the clitoris, the clitoris does not...

and, at least in theory, stops it from receiving direct stimulation, by acting essentially as a metal clitoral hood. However, because the shield is held in place by piercings (usually horizontal hood piercing), indirect stimulation will still take place.