Full Idea
Concepts, for Frege, are the ontological counterparts of predicative expressions.
Gist of Idea
Concepts are the ontological counterparts of predicative expressions
Source
report of Gottlob Frege (Function and Concept [1891]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
Book Reference
George,A/Velleman D.J.: 'Philosophies of Mathematics' [Blackwell 2002], p.20
A Reaction
That sounds awfully like what many philosophers call 'universals'. Frege, as a platonist (at least about numbers), I would take to be in sympathy with that. At least we can say that concepts seem to be properties.