Dihedral is the upward angle between the wings. On regular airplanes this is set permanently; but with paper airplanes the angle is a folded line and it can be easily adjusted.
The dihedral angle will eliminate inverted or spiral maneuvering during flight.
Because the paper is not stiff, when lift is applied on the wing during flight, it will fold up and increase the dihedral, so always set your dihedral angle lower than feel you will require!
Anhedral is a negative dihedral angle. It means the wings point downward! This will cause the paper airplane to turn upside down during flight, as a dihedral is always more stable situation for the plane.