A carabiner is a common piece of mountain climbing equipment, a metal clip that allows climbers to link together ropes and harnesses. If you're learning to rock climb, you'll most likely use a carabiner.
Carabiners are vital for climbers and mountaineers to keep various pieces of equipment connected to each other, and thus to keep themselves safe while they're climbing. A carabiner can take various forms and shapes, but it's usually a metal oval, with a spring-loaded clip that stays closed unless it's pushed open. Say the word carabiner with an emphasis on the bin syllable, but pronounce it like bean.