Harmonic rhythm is how often the music changes chords.
You know how when you sing "Mary had a little lamb", it sounds different when you start on a different note? The same thing happens in music when you change chords. It can make the music sound happy, sad, or any other feeling.
Harmonic rhythm is like how often we change our minds about what chord we want to play. If we change chords fast, like every one or two beats, it makes the music feel more exciting and restless. If we change chords slower, like every four or eight beats, it makes the music feel more calm and stable.
Imagine playing with a toy car and switching the direction all the time! Sometimes it's fun to go fast and make lots of turns, but other times you might want to slow down and just go straight. Harmonic rhythm is like that - it's about how many times we switch our musical direction.