Carbon offsetting is supposedly something everyone can engage in, both people and companies. You spend either your time or money on activities that try to reduce the global carbon dioxide levels. This is to compensate for (offset) any activities you have engaged in that increase CO2 levels, such as travel (especially air travel) and use of electricity produced by fossil fuels.
The most common offsetting activity is planting trees which will take in CO2 from the air as they grow and store it as trunks, branches and leaves. The following website is devoted to this cause: Trees for life.
However, you need to read their claims carefully as some scientists disagree -
see Carbon-info.org To make any difference to CO2 levels, the trees need to be planted in the right place. If we agreed a global CO2 allowance then you could buy carbon 'coupons' from someone (or another country) who is not using as much fossil fuel as you.