Full Idea
For Frege, concepts differ from objects in being inherently incomplete in nature.
Gist of Idea
Unlike objects, concepts are inherently incomplete
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
This is because they are 'unsaturated', needing a quantified variable to complete the sentence. This could be a pointer towards Quine's view of properties, as simply an intrinsic feature of predication about objects, with no separate identity.