RCMIP is about reduced-complexity, simple climate models and emulators. It focusses on testing and comparing their ability to emulate a range of CMIP6 coupled models. This Reduced Complexity Model Intercomparison Project (RCMIP) is hence not one of the standard "MIPs" that are part of the Sixth Coupled Model Intercomparison Project's (CMIP6) (see CMIP6 protocols by the World Climate Research Programme here).
Summary

Assessing how humans change the climate is a complex task, best investigated by complex Earth System Models. However, coupled atmosphere-ocean-biogeochemistry models are computationally expensive. Thus there is a need for emulators that are able to replicate some aggregate response characteristics of Earth System Models at a fraction of the cost. With such emulators, we can investigate uncertainties and simulate hundreds of possible future emission scenarios, rather than only a handful.

These emulators, ranging from one-line climate models to models with tens of thousands lines of code, are only useful if they can emulate more complex model results with a reasonable degree of accuracy. As a result, a systematic way to assess the ability of these emulators to replicate the results of complex climate models is required. This is what RCMIP is about.

RCMIP provides a standard protocol for one-line models, simple and reduced complexity models (henceforth we refer to the whole basket of models as RCMs) to be compared to the latest CMIP6 results. This provides a standardised test of their ability to replicate ESM projections for e.g. surface air temperatures, ocean heat uptake, gas cycles, effective radiative forcing and sea level rise.

Given the ongoing AR6 assessment cycle of the Intergovernmental Panel on Climate Change (IPCC), RCMIP envisages taking place over multiple stages. The first phase will run until mid November 2019, with a second phase planned for 2020. More phases can be added thereafter. Participation in RCMIP is open to all modelling groups who have peer-reviewed scientific papers that document their models or applications. All data submitted to RCMIP will be published on this website and will be published under an open access license (most likely Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)).

To our knowledge, RCMIP constitutes the first systematic intercomparison project among reduced-complexity climate models. The longest tradition of systematic intercomparisons is without doubt those by the coupled model community. Models of intermediate complexity (EMICs) have also performed systematic intercomparisons in the past. Reduced complexity climate models were often compared in the scientific literature, e.g. by van Vuuren et al. Climatic Change 2011. Nonetheless, RCMIP represents the first attempt to standardise this process in a systematic way.

More on https://www.rcmip.org/

EDIT: This might have been better under the Climate change in the news section. But too late I guess