וְשַׂמְתָּ֣ שָׁ֔ם אֵ֖ת אֲר֣וֹן הָעֵד֑וּת וְסַכֹּתָ֥ עַל־הָאָרֹ֖ן אֶת־הַפָּרֹֽכֶת׃
And thou shalt put therein the ark of the testimony, and thou shalt screen the ark with the veil.
Read kabbalistic (mystical) interpretation of Exodus 40:3: and classical mystical sources.