What would happen if they started spending all the money?
The money is invested and some of the yearly return on that investment is spent each year. At present the government is at liberty to spend up to 4% of the fund each year (though they don't always do this, due to risk of creating inflation, heating up the real-estate market etc). The investment fund is well-managed, but some argue that spending 4% each year might mean we're eating into the principle.