Jimping is the row of small machined grooves cut into the spine of a blade, the thumb ramp, or the front of the handle. Its only job is traction: it stops your thumb or forefinger sliding when you press down on the blade.
Most commonly on the spine directly above the handle, where your thumb naturally lands when you choke up for a controlled push cut. It also appears on the underside of the handle at the front, and occasionally along the top of a flipper tab.
Pattern varies from a few deep, widely spaced notches to a fine file-work texture. Deeper and wider grips harder; fine jimping is gentler on a bare thumb over a long session.
For wet work, gloves, or anything where you are leaning on the spine, jimping is a genuine benefit. For long cutting sessions with a bare thumb, aggressive jimping can raise a hot spot or a blister inside twenty minutes.
Try running a thumbnail across it before buying. If it feels like a rasp on a nail, it will feel like a rasp on skin after a hundred cuts.