The angle of attack is the angle created by the difference between the chord line of an airfoil and the relative wind. The chord line of a wing is an imaginary line drawn from the center of the leading edge to the trailing edge. Relative wind is what the wing feels as it travels through the air. Most gliders are designed with the optimum angle of attack achieved at trim or with no extra input on the glider. The angle of attack can be changed by the pilot through use of the brake toggles (brakes) or the speed system. As a non-powered aircraft our deviation from the mean angle of attack is relatively narrow. You should be careful and thoughtful about your gliders angle of attack for both safety and efficiency.
The chord line and angle of attack change with brake input, or when the wing suddenly encounters a vertically moving column of air. As the angle of attack increases, the amount of lift also increases, up to a point (minimum sink). If the angle of attack is increased beyond “minimum sink” the amount of lift decreases while drag increases until the airflow over the wing can no longer remain attached to the top surface. The lift force is instantly zeroed and the drag vector, in opposition to the flight path, becomes perpendicular to the horizon.
Beyond the stall angle, the glider is no longer generating lift and the only force slowing your descent is drag.