see more in this Forbes article

EDIT:

Several U.S. states and countries have Zero Emissions Vehicle regulations that require that clean vehicles account for a certain mix of auto manufacturers’ sales each year. If automotive companies, which still largely sell internal combustion engine-based vehicles, don’t meet these standards, they can buy credits from the likes of Tesla that earn credits, as they only sell electric vehicles. Although the revenues from these credits are quite volatile they are very lucrative for Tesla, as it likely incurs no direct costs to earn them.