If your car is an insurance write off they are basically saying the repair is worth more than the value of the car i.e Cat C say for example. Let's say their valuers think the car was worth £1000, this is decided by the make, model, mileage and condition of the car; your insurance company will give you £1000 BUT you want to keep the car too so in effect you buy the car from them for salvage/repair let's say that costs £150, so now they're giving you £850 + And you have to pay to have it checked to be safe to go back on the road.
I've never had to make a claim or anything (touch wood, touches head) that's from memory of my mate's car