According to Saussure, the signified is the conceptual element of the sign.
It is not the real object denoted by a sign -- the referent -- but a psychological entity corresponding to such an object.
Primacy of the Signifier
For Saussure, the signified has the same status as the signifier; both form equal sides of the sign.
Lacan, on the other hand, asserts the primacy of the signifier, and argues that the signified is a mere effect of the play of signifiers, an effect of the process of signification produced by metaphor.
In other words, the signified is not given, but produced.
Materialism of Language
Lacan's view is thus opposed to an expressionist view of language, according to which concepts exist in some pre-verbal state before being expressed in the material medium of language.
In contrast to such a view, Lacan asserts the priority (logical rather than chronological) of the material element of language.