Guitar String frequency
The fft provides the spectrum of the audio signal, which tells you the frequencies your signal is made of. In the case of harmonic signals like guitar, violin and other musical instruments, the spectrum exhibits peaks that are (almost) equally separated. For example, 110, 220, 330, 440... Hz. These peaks are called harmonics and they have different amplitudes, which contribute to the perception of timbre. 

The perceived note (aka the pitch) depends on the frequency of the first peak, that is the fundamental frequency. It could happen that the fundamental frequency, which is what you want to retrieve with your tuner, is not the strongest one in terms of amplitude. For this reason, you need more complex techniques than just retrieving the maximum of the spectrum. I suggest you to google pitch detection, which is the problem you are working on. There is a lot of material and possible solutions that you can compare and implement.