The acronym "אדמו"ר ריי"צ" stands for Admor Rayatz (אדמו"ר ריי"צ), which refers to Rabbi Yosef Yitzchak Schneersohn, the sixth Rebbe of Chabad-Lubavitch. The letters ריי"צ are an acronym for Rabbi Yosef Yitzchak (רבי יוסף יצחק), and he is commonly known by this title.

Rabbi Yosef Yitzchak was born in 1880 and led the Chabad movement through a time of tremendous upheaval, including the Russian Revolution and the difficult years of Soviet oppression. He was a strong advocate for Jewish education and observance, even in the face of persecution, and played a key role in the underground efforts to sustain Jewish life in Russia.

In 1940, he moved to the United States, where he established the Chabad-Lubavitch movement in North America and laid the groundwork for its global outreach. His leadership significantly shaped the future of Chabad, and his writings, teachings, and letters remain a vital part of Chabad Chassidus.