"The chromatic scale is a musical scale with twelve equally spaced pitches, each a semitone apart. A chromatic scale is a nondiatonic scale having no tonic due to the symmetry of its equally spaced tones."

basically what this is saying is that in western music we have divided an octave (the space between a frequency and exactly double that frequency) into twelve "pitches" that we call notes. non-diatonic refers to the fact that it has no basis in any of the western minor or major scales (the two fundamental western scales) as it covers every note of the octave, thus is circular and beyond definition.

Hope that helps