When companies earnings continue to grow so does the stock price. Public Companies are required to report earnings on a quarterly basis. Most larger companies have analysts that track the companies performance and make estimates on upcoming earnings. A composite of all the earnings make up the earnings estimates.
Whisper numbers are the numbers the street is looking for.
Although nothing is guaranteed in the stock market more times than not if a stock beats its earnings consensus estimate most likely the stock will increase in value. The opposite is also true, the market seems to often punish companies which miss their earnings number. Knowing the whisper number on the street helps in determining possible short term fluctuations in the stock price. Because of the very nature of a whisper number it is in fact a rumor. Market rumors may prove to be true.