raju1234:
It does not discuss about the complex Fast Fourier Transform at all.
The Fast Fourier Transform (FFT) is a rapid implementation of the Discrete Fourier Transform (DFT). In general the DFT takes a set of complex samples and creates a set of complex Fourier coefficients which completely describe the original set of complex samples and from which the original set of complex samples can be fully re-constructed. The inversion can be easily formulated in terms of the DFT.
Having collected a set of complex samples and transformed them using the FFT what you do with the Fourier coefficients depends on the application which you have not specified.
You need to understand complex arithmetic and understand Fourier transforms and then decide how they relate to the problem you are trying to solve. For this Google is your friend.