Forward lean is the angle the boot's cuff tilts forward from a straight-up position. Think of it as how much the boot naturally pushes your knees into a bent stance. Most snowboard boots come with a fixed forward lean between 12 and 18 degrees, though some high-end models let you adjust it.
More forward lean puts you in an aggressive, athletic stance with your knees bent, which helps with quick edge-to-edge transitions and responsive carving—great for freeriding and steep terrain. Less lean keeps you more upright and relaxed, which many freestyle and park riders prefer for pressing and spinning.
When choosing, consider your riding style. If you charge groomers or ride backcountry, more lean feels natural and responsive. If you're mostly in the park or prefer a surfy feel, less lean is the way to go. Just remember: your bindings also have a forward lean adjustment (the highback), so you can fine-tune your stance there as well.