As far as I know it just means your car has some metal hooks that are part of the car that a car seat can be attached to. They are under the back seat at the point where the back joins the bottom. We got an britax isofix car seat a couple of years ago as it was rated the safest in the Which and AA car seat tests. Quite expensive but they are very easy to fit and you know that they are in right which is something I was never sure about with our previous seat.
As Anchovies said, it is built into the car's framework so it's much safer than using seat belts to hold a car seat in position as it will not loosen or need checking once seat is in it's in until released and it is very easy to use, also isofix car seats can also be used in other cars by using the seat belt.