The MACD Histogram is similar in concept to the standard MACD except the histogram normalizes the signal or reference line to zero. This helps solve the lagging problem of the standard MACD.
The same principals apply here. Buy signals are generated when the MACD crosses over the zero line while sell signals are given when the zero line is crosses to the downside. Also when a stock moves up and the MACD Histogram fails to confirm the move, a divergence occurs. This signals a pullback may be coming. And when a stock moves lower while the MACD Histogram flattens out and moves up, a near-term bottom is likely.