i-Size is an European safety standard for baby and child car seats. It's part of regulation ECE R129, which will eventually replace the old safety regulation R44/04. The idea is that all car seats will eventually meet the tighter i-Size safety standards.
Your car must have Isofix connectors for you to be able to use an i-Size car seat. At the moment there are only a few i-Size seats on the market in the UK and not all cars have Isofix connectors.