Understanding FFT Results and the Importance of Replacing f0 in DFT Calculation

[VooDoo]

I got some data which is in equal time stamps. When I do the Fast Fourier Transform for the data using MATLAB I get the graph below. Now I understand how to do the fast Fourier transform but I have no idea what information the FFT graph gives.


I am confused as to what the peak at 1 represents and what else I can gather from the FFT?

 

[nada]

I would be so grateful if you could help me with
numerical Fourier of bessel function using FFT (with c if possible )or just the idea
also why we should replaced f0 by 1/2(f0 +fn) before calculatin DFT