Economies and markets tend to move in cycles, and any stock market can have a downturn once in a while. Most investors lose money when the stock market goes down, but some people may think they can time the market and gain.
The problem with this type of reasoning is that it’s impossible to know for sure when the market hits bottom. Most investors panic when the market starts to decline, then they decide to wait and end up selling after they have already lost considerable value. Or, on the recovery side, they buy in after the initial surge in value has passed and miss most of the upward momentum.