Rosen points out that art, especially music, has an inherent freedom and fluidity of meaning. Society lives by accepted mutual conventions, but in art these conventions can be subverted. The archetypal musical example is the deceptive cadence: everything is prepared for a normal cadential conclusion to the harmonic progression, but instead of ending up on the tonic, we end up on the submediant. Rosen's examples are more complex, of course. He cites passages in Mozart and Beethoven that exemplify in music the urge for political freedom that was so strong towards the end of the 18th century.
A lot of the essay is about the divergence between style and idea, between the signified and the signifier. In music you can write a simple dance that does nothing but exemplify the essentials of the form--but composers worth their salt usually do a lot more. The music takes on a life of its own, a playful or profound flourishing of the simple dance into something more. There are a host of examples: the Alla danza tedesca from Beethoven's String Quartet op 130 in B flat is a pretty good one:
Or the music could furnish an ironic version--not a subtext, because there is nothing "sub" about it--but an expression that is really at odds with the form. Shostakovich is the first place to look. Take the sardonic waltz that is the second movement of his Fifth Symphony:
But go and read the whole essay as a brilliant example of what music criticism is at its best.