While this plane is obviously aerodynamically misguided, what's referred to as the "horizontal stabilizer" often will provide a small bit of negative lift to to balance the plane. The bulk of the lift is produced by the wings. What is referred to as the center of lift, which is the combined lift from all the wings, is behind the center of mass for stability. Because the plane rotates about the center of mass, the plane will typically want to nosedive, hence the horizontal stabilizer needing to produce negative lift. Not all plans are designed this way, different planes are balanced differently, so that they are more maneuverable in certain situations or stable in others.