Full Idea
Russell's reasons for saying that definite descriptions are not referring expressions are: some definite descriptions have no referent, and they cannot be referring when used in negative existential truths, or in informative identity sentences.
Gist of Idea
Definite descriptions fail to refer in three situations, so they aren't essentially referring
Source
report of Bertrand Russell (On Denoting [1905]) by Mark Sainsbury - The Essence of Reference 18.5
Book Reference
'Oxford Handbook of Philosophy of Language', ed/tr. Lepore,E/Smith,B [OUP 2008], p.411
A Reaction
The idea is that by 'parity of form', if they aren't referring in these situations, they aren't really referring in others. Sainsbury notes that if there are two different forms of definite description (referential and attributive) these arguments fail.
Related Idea
Idea 7754 Russell's theory explains non-existents, negative existentials, identity problems, and substitutivity [Russell, by Lycan]