Well, if you are thinking of what I am, that bending is related to the production of lift. While on the ground, wings droop because of their weight. When the aircraft takes off, the wings produce lift and are then bent upwards. Then when the aircraft lands, the wings stop making lift and they bend down again, but not as much, because of their lighter weight (less fuel).
If you don't understand, take a 2x4, put a weight in the middle, and lift up on both ends. The 2x4 (wing) now has lift (made by you), and because of the weight of the fuselage (weight in middle), the ends bend up.