A genital clamp is a pinching device worn on the genitals either before or during sex. Men and women can both wear genital clamps and they are usually used as a form of sexual pain/pleasure during BDSM play.
The purpose of genital clamps is to increase sensitivity. For women, placing the genital clamps on the clitoris makes it much more sensitive and accessible which intensifies her pleasure.
The clamps can also be applied to her labia. The only safe place for men to use genital clamps is to the skin of the scrotum.
There are different variations of genital clamps. Many of them are spring loaded metal clips with rubber tips to protect your skin. Others are attached to nipple clamps with chains.
The choice is all yours, though I wouldn't splurge out on a pair to begin with. It's best to test the sensation on a cheaper pair first to see whether it's for you!