How did Frege explain the difference between “Venus is the morning star” and “Venus is Venus”?

Gottlob Frege explained the difference between informative identity statements and trivial ones by distinguishing between an expression's referent (Bedeutung) and its sense (Sinn).12

While "Venus" and "the morning star" share the same referent—the planet itself—they possess different senses. The sense of an expression is its "mode of presentation," or the specific way it presents the referent to the mind.12

• "Venus is Venus": This is a trivial, analytic statement because the name "Venus" presents the object in the same way on both sides of the identity, contributing the same sense to the thought.2
• "Venus is the morning star": This is informative because the two terms present the same planet through different "modes of presentation". Because these terms have different senses, they make different contributions to the thought expressed by the sentence.12

Frege argued that this distinction is essential to understanding how language can convey new knowledge about the world, rather than merely stating that an object is identical to itself.2