This is a oscillator calculated by the action on each bar price movement over a period of time.
When the short term (green line) is above the long term line(red)it indicates is coming and buying pressure may be observed.
Similarly when the short term is below the long term, is getting liquidated and selling pressure may be observed.
This indicator alone can't be used for buying or selling decision. This indicator can be used along with price action break out or break down level to get an idea whether is supporting the break out/down or not.
Disclaimer: Backtest this indicator with price action over a period of time and understand how it works before taking any trading decision. Trading in equity is always risky. Asses your risk profile and trade by managing proper risk.