More specifically, llAtan2(y,x) returns the angle between the 2d vectors <1,0> and <x,y>, just like standard C math library. The angle return ranges -PI to PI.
Object: llAtan2(0.000000,1.000000) = 0.000000
Object: llAtan2(1.000000,1.000000) = 0.785398
Object: llAtan2(1.000000,0.000000) = 1.570796
Object: llAtan2(1.000000,-1.000000) = 2.356194
Object: llAtan2(0.000000,-1.000000) = 3.141593
Object: llAtan2(-1.000000,-1.000000) = -2.356194
Object: llAtan2(-1.000000,0.000000) = -1.570796
Object: llAtan2(-1.000000,1.000000) = -0.785398