If you learn English then you will know the words "even" and "odd". Second, fourth, sixth etc. are even. Third, fifth, seventh etc. are odd.
A pure sinewave has no harmonics, it is the fundamental frequency only.
An amplifier stage with a sinewave input but has even harmonics added to the output has unsymmetrical gain so that the positive excursions or the negative excursions of the waveform are compressed.
An amplifier stage with a sinewave input but has odd harmonics added to the output has symmetrical compression of the output.