  We report low-frequency radio observations of the 2021 outburst of the
recurrent nova RS Ophiuchi. These observations include the lowest frequency
observations of this system to date. Detailed light curves are obtained by
MeerKAT at 0.82 and 1.28 GHz and LOFAR at 54 and 154 MHz. These low-frequency
detections allow us to put stringent constraints on the brightness temperature
that clearly favour a non-thermal emission mechanism. The radio emission is
interpreted and modelled as synchrotron emission from the shock interaction
between the nova ejecta and the circumbinary medium. The light curve shows a
plateauing behaviour after the first peak, which can be explained by either a
non-uniform density of the circumbinary medium or a second emission component.
Allowing for a second component in the light-curve modelling captures the steep
decay at late times. Furthermore, extrapolating this model to 15 yr after the
outburst shows that the radio emission might not fully disappear between
outbursts. Further modelling of the light curves indicates a red giant
mass-loss rate of $\sim 5 \cdot 10^{-8}~{\rm M_\odot~yr^{-1}}$. The spectrum
cannot be modelled in detail at this stage, as there are likely at least four
emission components. Radio emission from stellar wind or synchrotron jets is
ruled out as the possible origin of the radio emission. Finally, we suggest a
strategy for future observations that would advance our understanding of the
physical properties of RS Ophiuchi.
