This is the so called Momentum Indicator. The advanced version here is traded with the Forex Trading Framework that we coded in the MQL5 Premium course. It does trade based on the indicator in the lower window and it can process several different positions.
Wikipedia explains that a buy signal is generated when an up crossing through zero happens. In the other case when it crosses the zero line downwards, that would be a sell signal.
We are going to create entry signals based on the value in this case.