וְנָתַתָּ֙ אֶת־הַכַּפֹּ֔רֶת עַ֖ל אֲר֣וֹן הָעֵדֻ֑ת בְּקֹ֖דֶשׁ הַקֳּדָשִֽׁים׃
And thou shalt put the ark-cover upon the ark of the testimony in the most holy place.
Read kabbalistic (mystical) interpretation of Exodus 26:34: and classical mystical sources.