Probably the most significant thing about Moishe is that Eliezer bonds with him over spiritual matters. No one else in Sighet fulfills this role for him. Though Moishe is merely the jack-of-all-trades at the synagogue rather than a rabbi, he's versed in the Kabbalistic works—the Jewish writings regarding mysticism. It's the secrets of mysticism that Eliezer wishes to learn through Moishe.
We're told that Moishe is "physically as awkward as a clown." He's also the poorest of the poor in Sighet. In other words, he's an outsider in conventional terms, but perhaps his otherness is why Eliezer values his friendship and his mentoring. Moishe represents a different world that is not only beyond secularism and the bourgeois milieu of Eliezer's family, but beyond religion in its usual dimensions among most European Jews of that time.
Moishe is also a foreigner, not a native of Hungary, and because of this, he's deported before the others. He escapes the killings of those in his deportation group when he is wounded and left for dead, and he manages to return to Sighet. But no one there believes him when he tells them that the Jews are being systematically massacred. It is, of course, only a short time later that the entire Jewish population of the town, as in all of Hungary and the rest of Europe, are rounded up and deported to Auschwitz and other concentration camps.
Moishe is the Yiddish form of Moshe in Hebrew: Moses. For this reason, his name is symbolic. His act of warning the other Jews of what awaits them is a kind of metaphorical reenactment, though an unsuccessful one, of leading the Jews to safety, as Moses did the Israelites in the Torah. Through his character, Wiesel thus depicts the Holocaust as the tragic reversal of the escape from bondage of the Jewish people in antiquity.