Explanation of binning in frequency analysis post-FFT
In its simplest form, the 'binning' process consists in summing the energies (squared magnitude) within groups of adjacent FFT values. This will give you the total energy in a set of disjoint frequency bands.

A more elaborate form consists in using overlapping triangle filter banks - you compute a weighted sum of the energy in a range of FFT bins to get a number which can be interpreted as the energy measured at the output of a band-pass filter of a given center frequency/width. For example, one can use this technique to build a visualization which uses a 1/3 octave scale on the frequency axis.