The idea
The CAP eats up half our EU contribution. For this we get, well nothing really, but farmers get huge subsidies to not grow stuff which is in surplus.
The effect of the CAP is that farmers have an incentive to not improve their productivity, as it paid on acreage the more land your own the more subsidy you receive, so the richer get more. It reduces the retail price of food and so undercut growers in the third world, including the Commonwealth, and the same time as distorting the market.
Why is it important?
We cannot afford this any more.