Carbon credits is a scheme aimed at reducing greenhouse gas emissions. It is based on the idea of the 'polluter pays' according to how much pollution they generate. This is sometimes known as carbon offsetting.
The diagram below shows the process of the scheme.
The problem with carbon credits/carbon offsetting is that experts say that it does not really work. To be effective, oil companies and airlines need to stop carbon dioxide from entering the atmosphere, instead of trying to eliminate them after they put them there.