Complex sentence: Pirkei Avot is unique in that it is the only tractate of the Talmud dealing "solely" with ethical and moral principles; there is little or no halacha found in Pirkei Avot.
Split into simpler sentences: Pirkei Avot is unique in that it is the only tractate of the Oral Law solely dealing with ethical and moral principles. There is little or no halacha found in Pirkei Avot.