It's a a simple concept. If in one year we cut down trees faster than they can grow, fish the rivers, seas and oceans of fish stock faster then they can be replenished and generate more waste than the earth can absorb in the same year, then we have overshot that years target.
In 1987 ecological Overshoot was at the end of December, in 2007 it was at the beginning of October so at the moment we are using up the Earth's resources 30% faster than the Earth can possibly regenerate. Forget about nuclear power or biodiesel being the answer to our problems, the Earth resources are being liquidated and we are selling the family silver in order to pay for our standard of living.
Like the debt crisis we see today in the sub prime market and credit card problems, one day the bankruptcy of God's creation will surely follow.