Lamedvavnik is the Yiddish term for one of the 36 humble righteous ones or Tzadikim mentioned in kabbalah or Jewish mysticism. According to this teaching, at any given time there are at least 36 holy persons in the world who are Tzadikim. These holy people are hidden; i.e., nobody knows who they are. According to some versions of the story, they themselves may not know who they are. For the sake of these 36 hidden saints, God preserves the world even if the rest of humanity has degenerated to the level of total barbarism. This is similar to the story of Sodom and Gomorrah in the Hebrew Bible, where God told Abraham that he would spare the city of Sodom if there was a quorum of at least 10 righteous men. Since nobody knows who the Lamedvavniks are, not even themselves, every Jew should act as if he or she might be one of them; i.e., lead a holy and humble life and pray for the sake of fellow human beings. It is also said that one of these 36 could potentially be the Jewish Messiah if the world is ready for them to reveal themselves. Otherwise, they live and die as an ordinary person. Whether the person knows they are the potential Messiah is debated.
The term lamedvavnik is derived from the Hebrew letters Lamed (L) and Vav (V), whose numerical value adds up to 36. The "nik" at the end is a Russian or Yiddish suffix indicating "a person who..." (As in "Beatnik"; in English, this would be something like calling them "The Thirty-Sixers".) The number 36 is twice 18. In gematria (a form of Jewish numerology), the number 18 stands for "life", because the Hebrew letters that spell chai, meaning "living", add up to 18. Because 36 = 2×18, it represents "two lives".
In some Hasidic stories, disciples consider their Rebbes and other religious figures to be among the Lamedvavniks. It is also possible for a Lamedvavnik to reveal themselves as such, although that rarely happens—a Lamedvavnik's status as an exemplar of humility would preclude it. More often, it is the disciples who speculate.
These beliefs are articulated in the works of Max Brod, and some (like Jorge Luis Borges) believe the concept to have originated in the Book of Genesis 18:26:
And the Lord said, If I find in Sodom fifty righteous within the city, then I will spare all the place for their sakes.
Read more about this topic: Tzadikim Nistarim