Programs or policy regimes in which companies or individuals pay for activities that result in emissions reductions or CDR. In voluntary offset programs, individuals or companies pay project developers (or similar) directly to implement some activity that results in emissions reductions or CDR. In compliance offset programs, such as cap-and-trade programs, companies that are responsible for large amounts of emissions are allowed to continue to emit above a certain cap in exchange for projects taking place elsewhere that reduce emissions or remove carbon. In a compliance regime, an offset has no effect on total emissions in the best-case scenario and will result in more emissions than would have occurred otherwise if the project is ineffective in any way (e.g., due to failures of additionality or permanence).
source: J Wilcox, B Kolosz, & J Freeman (2021) CDR Primer; Concepts
The idea of carbon offsets arose from the Kyoto Protocol as an early way to initiate progress toward net-zero emissions; but is not thought to be a feasible model for actually achieving that goal.