This is one method of making sure the energy-suck of hosting is environmentally friendly. Purchasing a “carbon credit” essentially means the web host has calculated their total energy usage and purchased a credit for the greenhouse gasses they have contributed. Usually this means they are purchasing green energy and putting it into the energy grid.
This could also mean the host has contributed to a program that is supposed to have “scrubbed” the atmosphere of the amount of CO2 the host put in.
Important note: This does not mean the servers are powered by wind, solar, etc. This means the energy could have come from anywhere, and the host has made the effort to neutralize their carbon footprint.