Carbon intensity is a way of measuring the amount (in tons) of CO2 emissions within the context of economic activity. This can be measured at various levels:
For a country, GDP serves as the unit of economic activity (tons of CO2 emissions per million dollars GDP).
For businesses and industries, sales provide that gauge (tons of CO2 emissions per million dollars of sales).
Carbon intensity is a metric that allows us to standardize how we look at companies of vastly different sizes, or in vastly different industries.