Full Idea
Frege held that "and" and "but" have the same 'sense' but different 'tones' (note: they have the same truth tables); the sense of an expression is what a sentence strictly and literally means, stripped of its tone.
Gist of Idea
Frege's 'sense' is the strict and literal meaning, stripped of tone
Source
report of Gottlob Frege (works [1890]) by Alexander Miller - Philosophy of Language 2.6
Book Reference
Miller,Alexander: 'Philosophy of Language' [UCL Press 1998], p.56
A Reaction
It seems important when studying Frege to remember what has been stripped out. In "he is a genius and he plays football", if you substitute 'but' for 'and', the new version says (literally?) something very distinctive about football.