When a helicopter flies forward, the rotor blades generate unequal lift. This phenomenon is called "dissymmetry of lift".
For a clockwise rotating rotorhead, the blade on the left side of the helicopter is moving into the wind (advancing blade) and the blade on right side of the helicopter is moving with the wind (retreating blade). The advancing blade generates more lift, and the retreating blade generates less lift.
(Technically this is dissymmetry of moment, because the center of lift for the advancing/retreating blades is at different points along the length of the blade, but most most helicopter texts reference this as dissymmetry of lift.)
This dissymmetry of lift is equalized in different ways by the flapping and seesaw rotor heads.
For a flapping head, the upward motion of the advancing blade decreases the relative speed of the wind, and the downward motion of the retreating blade increases the relative speed of the wind. This is similar to holding your hand at the window at 40-50 mph. If your hand is slightly titled up at the leading edge, and you raise your hand, this decreases the apparent lifting pressure, and lowering your hand increases the apparent lifting pressure.
The flapping head may also use a rotor head where the two blades feather together (are rigidly connected on the feathering axis) and use blades where the center of pressure (center of lift) is behind the feathering pivot point. This causes the two blades to naturally equalize lift as the blade generating more lift will push its leading edge downward which also raises the leading edge of the other blade.
For a seesaw head, the advancing blade will rise up due to the extra lift. This rising motion causes the blade to feather and slightly decrease the angle of attack because it pivots around the blade grip's control ball. The amount of feathering is dependent on the angle formed by a line drawn between the blade grip control balls and a line perpendicular to the rotating axis of the blades, and is called the delta-three angle.
The delta-three angle may also be used with the flapping head to induce the blade to feather as it flaps up and down.
Note that this dissymmetry of lift is experienced by both the main and tail rotors on an R/C helicopter. The two rotor heads may use different methods of equalizing lift; for example, the ECO 8 uses a seesaw main rotor but uses a flapping tail rotor relying upon the delta-three angle to equalize lift.