Bitul Torah is from the severest aveiros; why isn't there an חטא על for this grave transgression? Sfas Emes zt'l answers that we say, חטא על דעת בבלי לפניך שחטאנו, "For the sin we committed before You without knowledge." This refers to when one committed aveiros because he didn't know that the Torah forbade his deeds. Ultimately, this is viduy for not learning Torah. If he knew Torah, he would know that this is forbidden, and he wouldn’t have committed those aveiros.
Furthermore, we say, לפניך שחטאנו חטא על עול בפריקת, "For the sin that we committed before You by throwing off the yoke [of Heaven]." The Sfas Emes writes that when one doesn't study Torah, this can be called casting off the yoke of Heaven.
Rebbe Reb Bunim of Pshischah teaches: We say in the zemiros of Motzei Shabbos, על חטאי, for my sins, תעבור עבור, we invite the King of the world to visit us. We will earn doubly: Hashem comes to us, and our sins are removed. This occurs on Yom Kippur. Hashem comes to us, and therefore, we are cleansed from all our aveiros. This seems to be the intention of the Mishnah (end of Yoma), 'ר אמר אתכם מטהר ומי מטהרים אתם מי לפני ישראל אשריכם עקיבא שבשמים אביכם, "Yisrael, you are fortunate! Before Whom do you purify yourselves? Before your Father in heaven!"
On Yom Kippur, we stand before Hashem, and that is the root of our purity and atonement. Rebbe Bunim explained it with a mashal: Someone tried very hard to clean his home, but he never succeeded. It was simply too hard a job for him to do because there was so much filth and dirt in his home. He came up with an idea. He invited the king to visit him, and the king accepted the invitation. Obviously, the king must go to a clean home. The day before the visit, the king's servants came to his home to clean it. The man earned twice. His home was cleaned, and he hosted the king.
