It is a nominally industry-led scheme to provide quality assurance for microgeneration products and installations.

Both the Feed-In Tariffs (for systems below 50kW) and the Renewable Heat Incentive (for systems below 45kWth) require that that the major equipment is product-certified and that the installers is accredited.

For MCS-accredited products see here. For accredited installers see here or, because they also have to be accredited under the REAL code,  here.