A crimp is both a hold (a small edge, typically less than a full pad deep) and the grip technique used to hold it. In a crimp grip, the fingers bend sharply at the second knuckle (the PIP joint) while the first knuckle (DIP) hyperextends backwards, with the thumb often locked over the index finger for extra grip — the "full crimp" or "closed crimp" position.
Crimping generates huge force on small holds but loads the finger pulleys (especially the A2 pulley over the second knuckle) heavily. Most pulley injuries happen during crimping. The "open crimp" or "half crimp" — fingers slightly bent rather than fully closed, no thumb wrap — is gentler on the pulleys and preferred for training and most climbing.
Progressively building crimp strength via hangboarding (open crimp first, full crimp only when strong) is the standard path. Crimps appear on every climb above V3 / 6c and become the dominant hold type as grades increase.