Sentence:  But what <WSD>is</WSD> equally impressive is the delicacy and wonderful lyric quality of both the band and Mulligan 's baritone sax in a fragile ballad like Bob Brookmeyer 's arrangement of `` Django 's Castle '' . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)