What does "Instantaneous Angle" means My teacher said to me that Instantaneous Angle means Argument of Complex Function thats all. Someone please tell me what significance does this "instantaneous" Word has here ?
Instantaneous usually means the value of something right at this time. I think what this means in your case is that the function $f(t)$ is complex and you are taking the argument (finding the angle):
$$
\theta(t) = \arg\left( f(t) \right) = {\rm atan}(\Im[f(t)],\Re[f(t)])
$$
where $\Im[ \cdot ]$ indicates taking the imaginary part and $\Re[ \cdot ]$ indicates taking the real part.