Green Certificate may also be known as a Renewable Energy Certificate (REC)
“A Green Certificate is a tradeable asset which proves that electricity has been generated by a renewable (green) energy source.” (Kyos.com)
Owners of renewable generation have one REC for each megawatt of generation. They can keep or sell their certificates. REC purchasers can claim that the energy they used came from a renewable source. Such purchases support renewable power generation.
The EPA has a video on RECs at this link: RECs: Making Green Power Possible.