A ton of carbon is (roughly) the same as a ton of coal -which you can probably visualise. Oil and natural gas (methane) also contain hydrogen, but most of their total weight is still carbon.
Burning 1 ton of carbon creates 3.67 tons of CO2 (3.67 is derived from 44/12 atomic mass units). As the global carbon cycle involves conversion between various forms of carbon (CO2, fossil fuel, plant biomass, bicarbonate ions in seawater, etc.), climate scientists find it more convenient to refer to tons of carbon than tons of CO2, but be careful which you are using when comparing figures.
Incidentally, 3.67 tons of CO2 is the same amount as is currently found in the whole column of air above 646 square metres of the earth's surface. Or it's about the same amount as the carbon stored in a typical tree about 20m tall. The energy you get from burning one ton of carbon (as oil) could light 30 60Watt lightbulbs continuously for one year.
Note, sometimes people refer instead to Gt CO2-equivalent (CO2-e) which includes other greenhouse gases scaled using "global warming potentials". Beware that the global warming potential for each gas depends on your time-horizon, since the gases have different lifetimes in the atmosphere!