The Angle of Incidence is the angle at which the aircraft wing is attached to the aircraft fuselage.
If you take an imaginary line from the center of the spinner, (the spinner is the cone attached to the propeller), to the center of the fuselage at the tail end of the aircraft, and extend the chord line of the aircraft wing, (flaps up), the angle where the two meet is the Angle of Incidence. This angle is usually 4°, which is the angle that offers the maximum amount of lift for the minimum penalty in drag.
This angle is also called the "Riggers Angle", because the wing was "rigged" or fitted onto the aircraft fuselage at this angle. When in straight and level flight, at cruise power, the aircraft will be at this "level" attitude when in flight, presenting a 4° angle of attack to the relative airflow. If the wing was to be attached parallel to the ground, the aircraft would have to reach a much faster airspeed before it would be able to become airborne, and would therefore require a longer runway. Not cool.
It can be quite alarming to witness how few bolts actually hold the wings onto the fuselage, but it seems to work. When mounting the wings, it is important to get the Riggers Angle equal on both sides. This is not an easy task to accomplish, and often small adjustments need to be made. When an aircraft is re-built or repaired, the wings are not always rigged perfectly, because of how time consuming this job is. The result is that you end up with an aircraft that has a tendency to roll to one side, or it develops a nasty tendency to flick to one side in an ordinary stall. When a training aircraft is properly rigged, it should have very docile stall characteristics. The play on each wing to adjust the angle of incidence is very little, and it is a very big job to adjust the wings. Correcting a rolling tendency can be done by having a flap adjusted slightly, or adding a fixed tab to one of the ailerons. Any qualified AMO (Aircraft Maintenance Organisation) should be able to correct a rolling tendency or wing drop. Rolling behaviour from an aircraft in straight and level or in a balanced climb or descent should not be shrugged off by the pilot.