what is the moral lesson behind the keystone in the mark degree

The moral lesson represented by the keystone—the stone which the builders rejected—serves to encourage those who are desponding 1. It conveys that even if a person's motives are misinterpreted, attainments underrated, or reputation maligned by others, divine judgment does not see with the eyes of men and may yet elevate that which was cast aside to become the head of the corner 1.

Regarding the history of this symbol in the ritual, F. Yates notes that the symbolism of the rejected stone was not part of the original Master Mark Mason's degree, but was introduced by Thomas Smith Webb from an unknown source when constructing his ritual 1.