This node creates a vector based falloff that associates a float to every vector. The float is equal to the normalized angle between the projection of two vectors on the plane defined by the inputs. The first vector is the positive x axis and the second vector is the vector in question.
The simplest case is when the input plane is the XY plane, in which case, the falloff value will be the normalized angle between the 2D projection of the vector and the positive x axis. By normalized we mean that the angle will have a range from 0 to 1 as opposed to from 0 to two Pi. This can be conceptually generalized to any plane in space by a 3D rotation.