Forward lean is the angle your binding's highback tilts toward your calf. It directly affects your riding posture and how quickly your board responds when you lean back on your heel edge. More forward lean pushes your knees into a bent, athletic position, while less lean lets you stand more upright.
If you're into freeriding or carving, crank the forward lean up (12-20°). The aggressive stance gives you instant heel-edge response and keeps you in a powerful position. For park and freestyle, go lower (0-8°) so you can ride more relaxed and tweak tricks without your legs burning out. Most all-mountain riders land somewhere in the middle, around 8-15°.
The key is to experiment—start in the middle and adjust from there. And don't forget to match the lean angle on both bindings, or you'll end up with uneven pressure that can throw off your riding and fatigue one leg faster.