Full Idea
Frege notes that an expression without a referent ('Pegasus') needn't lack a meaning, since it still has a sense, and the same referent (Eric Blair) can be associated with different expressions (George Orwell) because they convey different senses.
Gist of Idea
'Sense' gives meaning to non-referring names, and to two expressions for one referent
Source
report of Gottlob Frege (On Sense and Reference [1892]) by E Margolis/S Laurence - Concepts 1.3
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.4
A Reaction
A nice neat summary of the value of Frege's introduction of the sense/reference distinction, which seems to me to be virtually undeniable (a rare event in modern philosophy).