It calculates forward
based on your current savings rate (and a bunch of other assumptions) to find out how long your money will last under that plan, and also estimates backwards from your budget needs, accumulation years investment returns, and a
sustainable withdrawal rate to rough out how much you should be saving (
annually).
The present world population is consuming
annually almost one and a half times what the planet can provide
on a
sustainable basis, according to the WWF and Global Footprint Network.