Just a bit of background: Girard is paraphrasing Frege's famous paper On Sense and Reference[1] which is an investigation into the meaning of equality. As a result of that investigation, Frege shows that terms in a language have at least two kinds of meanings (sense and reference or denotation), which Girard presents in a programming context.
[1]: http://www.scu.edu.tw/philos/98class/Peng/05.pdf