You probably mean carbon credits, from the EU Emissions Trading System. Wikipedia has a lengthy and well-balanced article on the subject: https://en.wikipedia.org/wiki/European_Union_Emissions_Tradi...

It's an ugly and wasteful system set up instead of other, simpler measure that were politically unacceptable at the time, like higher VAT, excise duties on all fossil fuels across all industries without exception, including fuel oil heating and aviation fuel.