The more volatile a given equity instrumet, the larger its standard deviation. Standard deviation helps money managers to capture volatility of the portfolio into a single number. For most traded equities, future monthly returns are usually destributed within one standard deviation of its average return (68% of the time), and within two standard deviations 95% of the time.
Other deviation levels to watch out for are the 1.5 and 2 standard deviation level. At 2 standard deviations, the likely hood that your data point occurs within 2 standard deviations increases to roughly 95%. Again, just like any tool, this may not be 100% accurate, but it certainly have proven true more times than not. Using standard deviation is simple statistics and it takes emotion out of the picture. Another way people use standard deviation is to incorporate volume, which takes a little time to master the equation, but is certainly possible. Identifying what tools to use for you investing needs can take time, but a standard deviation tool is one to keep your eye on. It is reliable compared to the others and has proven to be one of the more useful out of the many that exist.