A Carbon Tax is a government imposed taxation on source goods and services in an attempt to reduce the production of carbon into the atmosphere.
Usually implemented as a form of 'at source' taxation; i.e. those whom manufacturer or consume the most carbon pay and everybody else on the supply chain picks up the price increase. This is often encountered with a carbon credit scheme, by which carbon producers can 'offset' their taxable carbon emissions against less carbon intense production methods.
As always with such schemes, the devil is in the detail; in particular there has been problems in the past with carbon fraud.
Note: This equally applies to the energy production sector as basic manufacturing; say if coal is being used as a source of electrical power.