Put another way – what is meant by a “rhythm section”? And more importantly, why is it called that?
A rhythm section is really just the drum kit and bass (upright or electric).
It’s called this because the rhythm section operates as an integrated unit by defining the rhythmic structure of the song as well as the tempo and the groove. That cool rhythm guitar part? Yeah that’s just filler or ear candy at best. The real song, the definitions of downbeat and the bass of the chord, are determined by this dynamic duo.
This idea is at the foundation of moving away from the pianist defining the song and the mix. And everyone has to be aware of this dynamic. Even if a song were to start out on acoustic piano, or with a guitar riff, as soon as the rhythm section is in, they define time.
When a rhythm section is working, it’s truly a beautiful thing. It frees other instrumentalists up to not have to “carry” the song, but to just play the parts that really add value.