Full Idea
Real functions map objects onto objects, but concepts map objects onto truth value, ...so Dummett says that concepts are not functions, but that they have a 'functional character'.
Gist of Idea
Concepts only have a 'functional character', because they map to truth values, not objects
Source
report of Michael Dummett (Frege Philosophy of Language (2nd ed) [1973]) by Donald Davidson - Truth and Predication 6
Book Reference
Davidson,Donald: 'Truth and Predication' [Belknap Harvard 2005], p.136