Some carbon credits operate by sequestration. Mainly from planting trees in a sufficient amount to offset a specified amount of carbon output.

"Each carbon credit represents one tonne of carbon dioxide either removed from the atmosphere or saved from being emitted.

Carbon credits were one of the outcomes of the Kyoto Protocol. They are a measure devised by the Kyoto Protocol to reduce world Greenhouse Gas emissions, and hence fight climate change. Carbon credits can be created in many ways but there are two broad types:

Sequestration (capturing or retaining carbon dioxide from the atmosphere) Or Carbon Dioxide Saving Projects

For each tonne of carbon dioxide that is saved or sequestered carbon credit producers may sell one carbon credit.

The carbon credits Carbon Planet provides come from tree sequestration projects, specifically Forests NSW."

taken from

https://carbonplanet.com/home/carbon_credits_faq.php