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Full Idea
In the 'Grundlagen' Frege takes the notion of the extension of a concept for granted as unproblematic.
Clarification
The 'extension' is the objects to which it applies
Gist of Idea
Early Frege takes the extensions of concepts for granted
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.16
Book Ref
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.200
A Reaction
This comfortable notion was undermined by Russell's discovery of a concept which couldn't have an extension. Maybe we could defeat the Russell problem (and return to Frege's common sense) by denying that sets are objects.