means those with the highest coin holdings (wealthiest) are the only ones able to sign blocks and transactions, being able to do it without requiring heavy work. The reasoning is that since they have the most coin wealth, they have the most to lose, and thus they won't try to screw the network, lest they crash the price of the coin and lose everything. The idea is also that they will have the most incentive to protect it.
Prof-of-stake sounds like a good idea only until you realize that it already exists, and is called USD and Federal Reserve.
It sounds like a bad idea. I heard of another one recently called ripple, ripple-project.org. A trust-based system but doesn't seem like a lasting idea.