A shear pin is like a fuse, but for a mechanical system. It's a part that's designed to fail first while being inexpensive and easily replaceable, so as to prevent the more expensive (and harder to replace) parts of the equipment from damage.

Last weekend, in the course of replacing the drive shaft on my bushhog, I discovered that just because I know better than to replace the specified Grade 2 bolt (that functions as a shear pin) with a Grade 5 bolt, doesn't mean that the last owner was similarly clueful. Fortunately no damage that I'm aware of, but I managed to shear the first bolt by hitting a relatively small stump (which I've tagged before), so I've either been operating outside of specified parameters or got unlucky with the cheap grade 2 bolts that I picked out of a "by the pound" bin.

Probably a good idea to have this on the checklist for buying used equipment, but also generalize to inventorying fuses on used vehicles and electrical equipment.