Handling the downside
The serious long term investor should not put himself in a situation where you will not survive a long period of falling stock prices. The last bear market in 2000-2003 lasted roughly 3 years, from the top around 1500 to the bottom around 800 a loss about -45%. One should be able to handle more downside than this and over a longer period. It may be crazy, but being able to handle -85% over 10 years. That would be pretty solid.

What is meant with handling is that a person will not loose his faith and sell when the world turns gloomy. It also means that he can handle the loss financially.

Limiting the downside risk
Being able to handle the loss financially means that the money riding in the stock market will not be needed during this period to pay for living expenses or any other situation that can arise. It is possible when the world’s economy takes a dive the investor may loose his job and a person must be able to make a living without the money invested.

Being able to take it financially means that your investment is not leveraged in such a way that you start to go into debt when the market goes down, getting margin calls and so on.

The payoff.
When the storm clears the market will rise again. The person that bought the SP500 at the very peak in 2000 is still in a little loss on paper. But the people that sold out and ran to hide lost the rebound.