Ars est celare artem means literally: the art is to hide the art, or in better English: the skill is in hiding the art.
Here the third declension noun ars, artis is given in the nominative (subject) and accusative (object) cases. Contrast that with ars gratia artis, where ars, artis is in the nominative and genitive cases.
ars, artis (f) art, skill
celo, celare, celavi, celatus to conceal or hide, disguise, keep in ignorance
