Full Idea
For Frege concepts are, precisely, the Bedeutungen of predicates.
Clarification
'Bedeutungen' in normally translated as 'references'
Gist of Idea
Concepts are, precisely, the references of predicates
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Crispin Wright - Frege's Concept of Numbers as Objects 1.iv
Book Reference
Wright,Crispin: 'Frege's Conception of Numbers' [Scots Philosophical Monographs 1983], p.15
A Reaction
On p.17 Wright challenges Frege's right to make that assumption.