A fantastic quote by Warren Buffett of Berkshire Hathaway.

“Be Fearful When Others Are Greedy and Greedy When Others Are Fearful”

What this means, is when there is a bubble, and asset prices are rising, and then prices are further fuelled by the shear volume and greed of more and more investors rushing in, with the herd mentality, then that is time to be worried and frightened.
When markets are tumbling, and all of a sudden certain assets seem to be oversold and undervalued as everyone is selling, that is the time to enter the market, be greedy and buy assets below their real value.