Full Idea
Wittgenstein's 'fundamental idea' is that the 'and' and 'not' which guarantee the truth of "not p and not-p" are meaningful, but do not get their meaning by representing or standing for or referring to some kind of entity; they are non-referring terms.
Gist of Idea
'And' and 'not' are non-referring terms, which do not represent anything
Source
report of Ludwig Wittgenstein (Notebooks 1914-1916 [1915], §37) by Robert Fogelin - Walking the Tightrope of Reason Ch.1
Book Reference
Fogelin,Robert: 'Walking the Tightrope of Reason' [OUP 2004], p.28
A Reaction
Wittgenstein then defines the terms using truth tables, to show what they do, rather than what they stand for. This seems to me to be a candidate for the single most important idea in the history of the philosophy of logic.