Full Idea
Contrary to Dedekind's anti-realism, Frege proposed a realist definition of a set as the extension of a predicate (or concept, or function).
Clarification
The 'extension' of the set is the actual things which the set contains (e.g. trees); hence he must accept real trees
Gist of Idea
Frege proposed a realist concept of a set, as the extension of a predicate or concept or function
Source
report of Gottlob Frege (works [1890]) by José A. Benardete - Metaphysics: the logical approach Ch.13
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.91