Answer: "Harmonic rhythm" refers to the rhythm of chord changes, as distinguished from the rhythm of a melody. Usually you don't hear about any specific rhythm, though: harmonic rhythm is most often described just as "slow" or "fast." But "static" harmonic rhythm would mean super slow: spending a long time on the same chord. You could also speak of syncopated harmonic rhythm, if the harmony were changing on offbeats and tying over to the downbeat. But more normally, harmony will provide the steady underpinning while the melody syncopates. Harmonic changes are a good clue to the true meter of a piece: harmonic changes are very rarely quicker than a beat, and usually no more than two to a measure in 4/4 of moderate tempo. Harmony tends to change on the strong beats, e.g. on 1, or 1 and 3 in 4/4.

Repeating the same harmony doesn't count as a chord change, of course - even if the chord is revoiced. To count as a change, the chord needs to change its root, its quality, or both.

So: "static" harmonic rhythm would be that of a passage that pretty much sticks to the same chord: if the same chord lasts for 4 measures of 4/4, for example, you could call that static harmonic rhythm. "Regular" harmonic rhythm would refer to chord changes that take place at regular intervals or in a regular pattern. "Fast" harmonic rhythm would be changes occurring once a beat or once every two beats or so in a quick tempo; "slow" harmonic rhythm might be changes of once a measure or once every two measures.