MACD is developed by Mr. Gerald Appel, which is combined the Oscillator analysis and the trend analysis. MACD is using the two index smoothing MAs, one is the leading curve and the other is delayed curve.
The cross point of those curves is regarded as the most significant signal to buy or sell in the MACD analysis. It suggests to buy when the leading curve is across the delayed one upward, and to sell when the leading curve is across the delayed one downward. It indicates the stronger sign when the leading curve crosses the delayed curve upper area than the central line, and vice versa.
In addition, the trend line on the index smoothing MA might show you the turning point of the current trend. You can choose the best one by tuning fine the preset period.