It's not really very complicated. In the manner of most media presentations of anything to do with money, it can be shown in chart form.
Imagine your standard of living as the curve on a graph. There will be all sorts of things moving it up and down like having children, redundancy and ill health, but the predictable biggy is retirement, which is potentially a big drop in that curve.
Anybody planning security in retirement, rather than day dreaming, has to think in terms of ensuring that the curve remains as flat as possible. Spending every penny as it comes in will not achieve that. Borrowing money while in employment will lift the current standard of living but cannot continue indefinitely, while risking increasing the drop in retirement. Saving a bit along the way can help keep the line level after retirement.
All manner of problems, of course, not least thieves in sharp suits breaking through and stealing the nest egg, Cap'n Bob being an extreme example of this.
I look back now on a life of being over-cautious in money matters, saving not only against a rainy day but against Noah's Flood. eg Our children never went on foreign holidays, unless you count Wales and Scotland as foreign countries. The result is that my curve is higher than at any time in my life, but I'm so cautious - close with money - I'm reluctant to spend it. Ebeneezer Scrooge? A total amateur.
