Dihedral is basically the little space between the wings of a plane or kite.
It is the angle on a plane's wing because it is the part where the wing and the plane are attached together so that is the angle (Dihedral).
For example: this is a plane Dihedral:
Another Example: A kite Dihedral:
It is just like a plane just that a kite is an aircraft consisting of many wings tethered to an anchor system and a plane is a powered flying vehicle with fixed wings.

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