p>On Yom Kippur we recite 53 different verses of “Al cheit,” repenting for various types of sins. The mouth and tongue are the limbs mentioned most often in “Al cheit”: utterance of the lips, through speech, insincere confession, foolish talk, impurity of speech, speaking evil about another, prattle (meaningless sounds) of our lips, tale bearing, swearing in vain… Why is there so much emphasis on the mouth?
Rabbeinu Yona explains: By guarding the mouth, it becomes a holy vessel. Once a holy vessel becomes defiled, the holiness completely departs from it and much work is needed to draw the holiness back into it anew.