Full Idea
For Frege, a thought is not something psychological or subjective; rather, it is objective in the sense that it specifies some condition in the world the obtaining of which is necessary and sufficient for the truth of the sentence that expresses it.
Gist of Idea
A thought is not psychological, but a condition of the world that makes a sentence true
Source
report of Gottlob Frege (works [1890]) by Alexander Miller - Philosophy of Language 2.2
Book Reference
Miller,Alexander: 'Philosophy of Language' [UCL Press 1998], p.33
A Reaction
It is worth emphasising Russell's anti-Berkeley point about 'ideas', that the idea is in the mind, but its contents are in the world. Since the contents are what matter, this endorses Frege, and also points towards modern externalism.