The yoke is the shaped panel of denim across the back of a pair of jeans, positioned above the back pockets and below the waistband. Rather than being cut as one flat piece, the back of the jeans is split here into two angled sections joined at a seam, which lets the fabric follow the curve of the lower back and seat more closely than a single flat panel could.
Because the yoke is cut on an angle relative to the grain of the fabric, it also plays a role in how the denim will stretch and fade over time, and many brands treat its shape and depth as part of their signature back pocket styling. A deeper, more angled yoke generally gives a closer fit through the seat, while a shallower yoke sits closer to a flat, boxier back.
The yoke is largely a construction and fit detail rather than something to actively choose, but it is worth knowing when comparing how different brands' jeans shape to the body. See our guide to jeans for different body types for more on how back construction affects fit.