This is a talk by the Rebbe, from the year 1967 (the original in Yiddish can be found in Likutei Sichot, vol. 29, p. 203) on Yom Kippur.
The talk revolves around the interpretation of Maimonides' words, in his Laws of Teshuvah, on the difference between the Teshuvah of the Ten Days of Teshuvah (from the first of Tishrei, Rosh Hashanah, and the 10th of Tishrei, Yom Kippur) and the Teshuvah of Yom Kippur.
Furthermore, Rambam mentions that Yom Kippur is "the time of Teshuvah for everyone," but in reality, if a person sinned, the obligation to do Teshuvah immediately falls upon him; he does not have to wait until Yom Kippur. On the other hand, if he did not sin, why should he do Teshuvah on Yom Kippur?
The class lasts approximately one hour.