Allow me to explain in small words.
Global warming started in about 1850 when the Industrial Revolution dramatically increased the burning of fossil fuels (coal and petroleum). That burning took carbon dioxide (CO2) that had been sequestered (trapped) in the ground and put it in the atmosphere.
When light from the sun shines on the surface of the Earth, it is absorbed there and warms the planet. The warm planet radiates away its heat as infrared light (IR). CO2 absorbs IR and slows its escape to space. On a planet with no CO2 in the atmosphere, that radiated IR would escape directly to space. If the Earth had no gases absorbing IR, the temperature would be almost 50 Fahrenheit degrees colder than it is. Instead, that warmed CO2 reradiates its energy as more IR which usually gets reabsorbed by other CO2 molecules and also transfers some of its thermal energy to other gases in the air as well as to the land and the ocean through direct conduction (contact). The IR energy eventually ends up either back in the Earth itself, or finally escapes to space. The more CO2 there is in the atmosphere, the longer it takes that IR to escape and the warmer the Earth will become.
From the time of the first appearance of humans 200,000 years ago till the Industrial Revolution, humans have lived on a planet with 280 ppm (parts per million) of CO2. CO2 released by burning coal and oil have now raised that value to 420 ppm. That additional CO2 in the atmosphere has caused the planet to warm significantly with more to come even if we were to immediately stop putting out CO2. That warming is having lots of different harmful effects that hurt plants and animals and humans in a variety of ways.
In a perfect example of the American Way, it has been proposed that businesses will do the best job of reducing and eliminating greenhouse gas emissions if we simply encourage them to move in that direction and then let them find their own means of doing so. That is the purpose of a carbon tax.
Questions?