It stands for Moving Average Convergence/Divergence.It is essentially the average of price with respect to some time interval and a means of showing differences between a fast EMA and slow EMA (Exponential Moving Average) at closing.A fast EMA is considered with a time sample of 12 days and a slow EMA with interval of 26 days.

MACD = EMA[12] of price - EMA[26] of price

EMA of 9 days , EMA[9] of price is considered as the signal line.It is plotted over the above MACD result.

If MACD falls below the signal line then bear or sell market else buy market.