The Momentum indicator is today's Close divided by the Close n-periods ago multiplied by 100. It is generally used to measure trend strength and identify possible trend reversals. The higher the indicator value (on the positive side of the zero line) the more aggressive the uptrend. Conversely, the lower the value (on the negative side of the zero line) the more aggressive the downtrend.
Values over 100 (the zero line on the oscillator) indicate the current stock price is above the price "x" (period selected) bars ago. Below that line, current price is lower than prior price. Values oscillating around the zero line (100 value) indicate no real trend is in place.
Some signals can be created either by placing a moving average on the indicator and looking for crossovers or using price/indicator divergence techniques.