Forward lean is a term for the angle at which the highback of your binding is set. Most bindings with high backs have an adjustable forward lean system on the rear of the high back, and the systems used vary among companies. Zero forward lean means your binding high backs are perpendicular to the binding base, and an increase in forward lean pushes your calves forward toward the toe edge of the board.
Forward lean is intended to give snowboarders a lower center of gravity for more power and precision in heelside turns. Not all riders feel that forward lean is helpful, but it’s an added bonus of binding construction for those who want to use it.