Sentence:  In the work of every artist , I suppose , there may be found one or more moments which strike the student as absolutely decisive , ultimately emblematic of what it <WSD>is</WSD> all about ; not less strikingly so for being mysterious , as though some deeply hidden constatation of thoughts were enciphered in a single image , a single moment . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)