After the octave, the interval of a fifth is the one that forms the foundation of harmony. The fifth is often considered neutral because it does not alter the color of a chord. The color of a chord is primarily defined by the third. It can be further shaped by additional notes, such as the seventh or ninth.
The purpose of this exercise is to help you distinguish a simple chord (based on intervals of an octave and a fifth) from a complex chord that includes a third, a seventh, or a ninth.
Remember that simple chords built on the octave and fifth sound less colorful (the neutral or 'perfect' intervals give a sense of unity and openness), while complex chords with a third or seventh sound more colorful (giving a sense of diversity and complexity).
