Full Idea
Kripke's main contribution was …his classification of the different consistent fixed points and the discussion of their use for discriminating between ungrounded sentences, paradoxical sentences, and so on.
Clarification
At a 'fixed point' a function outputs its input
Gist of Idea
Kripke classified fixed points, and illuminated their use for clarifications
Source
report of Saul A. Kripke (Outline of a Theory of Truth [1975]) by Volker Halbach - Axiomatic Theories of Truth 15.1
Book Reference
Halbach,Volker: 'Axiomatic Theories of Truth' [CUP 2011], p.210