The following is a simplified presentation of complex biophysical phenomena. Readers who wish to know more about this topic may refer to specific literature.
Since one century, the development of economic activity plays a very central part in the acceleration of the emissions of greenhouse gases. Production and consumption processes as well as waste management involve combustion activities, i.e. among other these activities releases carbon dioxide and water vapour.
In short, trees are responsible of the reverse reaction: photosynthesis. Trees capture CO2 and water and make, well, mainly wood and release oxygen in the atmosphere.
Well-managed forests are carbon wells. The mass of carbon captured by a tree depends on the mass of the tree itself. A rule of thumb can help to approximate the carbon stored in one tree: 1 m3 of wood is associated to the capture of 1 ton of CO2.
The annual carbon footprint of 5 tons of CO2 could be compensated by planting five trees, assuming these trees can reach maturity. This may take between 10 and 50 years depending on local condition.