The soul of every Jew is comprised of five distinct levels, ranging from the lowest aspect of the soul to the highest. The first four levels represent the soul's different modes of expression: action, emotion, intellect and will.
The fifth level—yechida—represents the soul's very essence. While every Jew possesses a soul comprising all five levels, it is Moshiach, the yechida of the Jewish people, who makes it possible for the essence of every Jewish soul to be revealed.
(Rabbi H. Greenberg in Di Yiddishe Heim)