Full Idea
Frege's theory treats existence as a property, not of things we call existent, but of concepts instantiated by those things. 'Biden exists' says our Biden-concept has instances. That is certainly not how it feels! We speak of the thing, not of concepts.
Clarification
'Biden' is an American politician
Gist of Idea
Frege mistakenly takes existence to be a property of concepts, instead of being about things
Source
report of Gottlob Frege (On Concept and Object [1892]) by Stephen Yablo - Aboutness 01.4
Book Reference
Yablo,Stephen: 'Aboutness' [Princeton 2014], p.16
A Reaction
Yablo's point is that you must ask what the sentence is 'about', and then the truth will refer to those things. Frege gets into a tangle because he thinks remarks using concepts are about the concepts.
Related Ideas
Idea 18752 'The concept "horse"' denotes a concept, yet seems also to denote an object [Frege, by McGee]
Idea 18899 Frege takes the existence of horses to be part of their concept [Frege, by Sommers]