MACD is the difference between two exponential moving averages, plotted with a signal line and a histogram. Traders buy when the MACD line crosses above the signal line, sell when it crosses below, or use crosses of the zero line and divergence from price.
The claim is that MACD captures momentum and its turning points earlier than the averages it is built from.
We tested MACD crossovers, zero-line crosses and divergence across 17 years of data with real trading costs, and found no edge. The signal-line crosses are frequent and mostly wrong in ranges; the zero-line crosses are a moving average crossover under another name, and share its lag.
The reason is that MACD is built entirely from moving averages, and inherits every property of them. It cannot contain information the averages do not.
If you trade it anyway, treat it as the crossover strategy it is, and see that page.