Commodity Channel Index
The CCI is the difference between the mean price of a commodity and the average of the means over the time period chosen.
This is then contrasted with the average difference over the time period and multiplied by a constant to fit it into a +/-100 range.
Although originally designed for commodities it works as well (or as badly!) with stocks.
You can use it to spot divergences (underlying making new high/low - CCI failing to surpass previous high/low) or as an overbought/oversold indicator (anything outside the 'normal' range of +/-100).
Should you use it? If it helps in your trading system, yes, of course.