Full Idea
In standard set theory, Frege's cardinals could not be members of classes, and his proof of the infinity of natural numbers fails. Nowadays they are defined as sets each representative of its cardinality, comprising ordinals of lower cardinality.
Gist of Idea
Frege's account of cardinals fails in modern set theory, so they are now defined differently
Source
comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.14
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.168
A Reaction
Pinning something down in a unique way is not the same as telling you its intrinsic nature. But a completely successful definition seems to have locked on to some deep truth about its target.