(gross oversimplification of music theory follows:) You've probably heard of complementary colors and the color wheel in the context of art. The idea there is

(gross oversimplification of music theory follows:) You've probably heard of complementary colors and the color wheel in the context of art. The idea there is (afaik) that some colors are thought to go "better" together and they're used more often together to create certain effects that seem more naturally fitting. If nothing else, there are color pairs used often together, by convention. Music has a similar construct in chord progressions. Chords (combination of pitches played simultaneously) sound as if they "naturally progress" from one to another when played in certain orders. For example, lots of pop songs will use a I-vi-IV-V progression over and over again, because 1) it's conventional and popular and 2) it sounds "natural" i.e. like it belongs together, the same way some colors might to an artist. What they mean here by _harmonically unexpected chord within a standard chord progress_ is that they started with a standard, conventionally popular progression of chords, then (I assume) replaced one (or more) of the chords that follow that progression with one that doesn't, which creates a feeling of dissonance since they aren't supposed to belong together, classically speaking. Hopefully this is an adequate portrayal -- not trained in theory, just a casual songwriter.