I-Size is a new European standard for child seats. It works along side the existing law ECE R44.04. The new law is only applicable to the new seats that would be manufactured, and does not apply to child car seats already being sold or that are in use. Hence there is no need to throw away the existing child seats if it is not i-Size approved.
These seats undergo additional test for side-impact and to access the protection if the vehicle is involved in a collision. It also ensures that the child is positioned in a rearward facing position as it is much more safer than the forward facing seats.
The i-Size seats are based on the height of the child instead of their weight.
You could check this YouTube video produced by the car seat manufacturer Maxi-Cosi, where they explain it more in length.
A i-Size seat will fit any i-Size approved vehicle. However at this point in time, as this law is fairly new, there are fewer cars that have a i-Size feature yet. Its natural to presume that in the coming years there would be more cars available that would be i-Size compatible.