fcenter is an approximated frequency of the strongest pitch detected from the frequency domain data obtained using FFT. The "neighbourhood" of this frequency is used as a basis for searching a more accurate estimate using a normalised autocorrelation algorithm. The final estimated pitch is fest.
The nearest chromatic note is shown on the tuning meter, along with the deviation from the note pitch in cents. "--" means that the signal level is too low to detect. "!!" means that a sufficiently strong signal exists but a reliable estimate for the pitch has not yet been acquired.
The detection modes incorporating FFT are best used with tones that have a clearly defined fundamental frequency. However, for detecting the pitch of human voice, it's best to use only autocorrelation as it seems to provide the best results with sounds containing a large amount of harmonic tones.