In a beam reach, the boat is sailing at a perpendicular angle to the wind. The wind is coming directly across the beam of the boat.
Notice that the sails are let out farther in a beam reach than when close hauled. The flow of the wind over the curve of the sail is, again, like air around an airplane’s wing, generating lift to move the boat forward.
Note too that the boat heels less than when close hauled.
All other factors being equal, beam reach is often the fastest point of sail for most sailboats.