"Rhythm By Addition" is a term in music theory that defines the metric style of non-Western music, whereby rhythms are built in layers, and are not subdivisions of the beat ('Rhythm By Division").
In African music, there are many independent rhythms happening simultaneously. In Western pop music rhythms are wrapped around the square box of 4/4.
Hip-hop was the first genre to break out of the rigid hierarchical structure of quarter notes dividing into eights and sixteenths, and "adding" additional rhythmic filigree over the top as a counterpoint.
To speak of 'counterpoint' is to relate it to melody only, with the vertical consequence of polyphony. The same thing happens with rhythms--namely, polyrhythms.
In pop music you can lock into a beat and stay there. In polyrhythmic music you can relate to disparate layers and still mesh with the beat.