The trailing edge is marked red on the above photo, the opposite is called leading edge.
The trailing edge is the edge over which the wind leaves or last touches the kite. Consequently, it is at the end of the canopy. Kite manufacturers usually deliberately reinforce this trailing edge / trailing edge with additional material and / or by folding the material over and using additional seams.
This is necessary to avoid stretching of the cloth and to improve the flight characteristics.