In a broad reach, the boat is sailing far off the wind (but not quite directly downwind). Note that in a broad reach the sails are let much farther out. The boom is far out to the side, and the jib loops forward of the forestay.
The shape of the sails is still generating some lift, but as the boat heads farther and farther off the wind, it is increasingly being pushed forward by the wind from behind rather than being pulled forward by lift.
Note also that the mainsail out to the side is almost directly behind the jib, in relation to the wind coming from behind. If this boat were sailing directly downwind, the mainsail would block the wind and keep so much wind from the jib that it would not fill. Most sailors therefore prefer to sail off the wind in a broad reach rather than directly downwind. A broad reach is faster, and there is less risk of an accidental gybe. A gybe occurs when headed downwind and a wind shift or gust throws the mainsail across to the other side, stressing the rigging and risking the boom striking someone as it crosses the boat.