offsets made simple
This is an awesome website:

Each time we heat our homes, take a flight or drive the car, CO2 is added into the atmosphere. CO2 is a greenhouse gas that is released when fossil fuels such as oil, gas and coal are burnt.

Offsetting means paying someone to reduce CO2 in the atmosphere by the same amount that your activities add. In this way you can 'neutralise' or 'balance' the CO2 added by your activities.
This site makes it easy - check it out here