Actually, arctan2(y, x) is to expand the range of arctan(y/x) from (-pi/2, pi/2) to (-pi, pi).

Or there is an equivalent statement: the angle between the vector whose origin points to the coordinate (x, y) and the positive direction of the X axis counterclockwise.

This function is generally used in computer numerical calculation, and many mathematical softwares record it as atan2(y, x).