Traction, and nothing else. Jimping is the row of small machined grooves on the spine of a blade, on a thumb ramp, or on the front of a handle. It stops your thumb or forefinger sliding when you press down on the blade during a controlled cut.
Most commonly on the spine directly above the handle, which is where your thumb lands when you choke up for a push cut. It also appears under the front of the handle and sometimes on a flipper tab.
The benefit is real with wet hands, gloves, or any job where you lean on the spine.
Deep, widely spaced grooves grip hardest and are hardest on a bare thumb. Fine file-work texture is gentler and less positive. Neither is correct; they suit different uses.
Run a thumbnail across it before buying. If it feels like a rasp on a nail, it will feel like one on skin after a hundred cuts.
Jimping is not decoration, though decorative spine file work looks similar and the two often overlap. It also does not weaken the blade meaningfully, because the grooves are shallow and sit in the thickest part of the steel.
See the glossary entry for the full definition.