The MACD is the difference between a 26-day and 12-day exponential moving average of closing prices. It is popular to buy/sell when the MACD goes above/below zero (known as a "centreline crossover").
Great momentum stocks stay above zero for a long period of time. Cross overs below zero are usually be ignored since the stock is weak and it is said that trends can not be predicted as easily.
You can read more about the MACD Strategy here.