While there are some good explanations already mentioned, the bottom line is that stock prices are forward-looking in the sense that investors buy and sell stocks not based on what happened yesterday or what is happening today, but rather based on their expectations for the future (6-12 months ahead).

So, basically, the market is signaling that on a whole (i.e. weighted average growth of all the companies in the S&P 500) things (i.e. revenue/eps) aren't going to get dramatically worse and potentially going to get increasingly better.