Full Idea
Frege moved from an extensional semantic theory (that countenances only linguistic expressions and their referents) to an intensional theory that invokes in addition a notion of sense.
Gist of Idea
Frege moved from extensional to intensional semantics when he added the idea of 'sense'
Source
report of Gottlob Frege (On Sense and Reference [1892]) by Sarah Sawyer - Empty Names 2
Book Reference
'Routledge Companion to Phil of Language', ed/tr. Russell/Graff Faria [Routledge 2015], p.154
A Reaction
This was because of Frege's famous 'puzzles', such as the morning/evening star. Quine loudly proclaimed himself an 'extensionalist', implying that he had extensional solutions for Frege's Puzzles.