A carbon tax is a fee associated with carbon emissions. This is most often in the form of a business burning fossil fuels. A business can burn fossil fuels directly (via their delivery fleet) or indirectly (the electricity that powers their store front originates from a coal fired power plant).
Carbon taxes are an economic way of incentivizing our transition away from fossil fuels.
Currently businesses who emit global warming gasses shoulder no cost for their actions. This and other externalities go unaddressed in the existing free market, which means there is no incentive for businesses to reduce their emissions.
By placing a tax on emissions, we will have an economic model that is more representative of the actual market.