Grammatically, there is nothing special about this phrasing. Any intransitive verb could replace 'is', and 'what' is a relative pronoun replacing the subject of the relative clause.
Jiddu knows what is.
Jiddu knows who remains.
Jiddu knows what jumps.
'is' is intransitive here, though usually as a copulative, expects a following subject. This may be your intuition about what comes after 'is'.
In
I am what I am.
the first 'am' has an following phrase, what I am'. The second one doesn't need or expect anything to follow.
Since 'is' is one of the most common words (in almost any language that uses copulatives (some don't)), it is very general, has lots of nuances in meaning in many different contexts, and therefore without further explanation is very vague.
In usual philosophical style, concepts which are new and abstruse are often explained with vague, non-specific vocabulary in an almost mystical way, striving to get the reader to fill in the opaque gaps. "What does 'is' mean? What is 'real', or 'actual', or 'wisdom'? The author thinks they know and being vague gives them a lot of freedom to be assumed to be profound. Or maybe he just grew up in a household where some parent was 'away for work' but nobody ever said anything and tiptoed around it, being all weird, and as a child he couldn't figure out why, and it turned out the dad was in prison. Why? Oh that's a whole nother story... that's what's really real.
Anyway, sometimes philosopher's get carried away. Sometimes they make up new words for existing concepts and make them obscure, and sometimes they use old words in totally new ways to make them obscure.
But without the full context of Krishnamourti's text, it is difficult to figure out exactly what 'is' means for him (if it is indeed different from the usual everyday 'is').