Gas, oil and coal all contain carbon - when they burn the carbon is oxidized to carbon dioxide.
Carbon dioxide, as we all know, enters the atmosphere, and is generally believed to cause global warming
There are various schemes to reduce the emission of carbon dioxide, called emissions schemes, and these require major emitters of carbon dioxide to provide certificates matching their emission of carbon dioxide. These certificates may be acquired in a number of ways, and there is a market for organizations with surplus certificates to sell, and organizations who need more certificates, to buy
Naturally wherever a market exists to trade anything, speculators attempt to profit by buying and selling - in this case - carbon (in the form of certificates)