MACD plots the difference between a 12-period and 26-period EMA, along with a 9-period EMA of that difference (the "signal line") and a histogram. Crossovers of the MACD line over the signal line are treated as momentum shifts; divergence between MACD and price is a classic reversal signal.
Like RSI, MACD works best in sideways or mean-reverting markets and can produce many false signals in strong trends. The best practical use is divergence — when price prints a new high but MACD does not, momentum is weakening even if the trend is still technically intact.