An MACD-histogram indicates the distance between MACD and its signal line (the 9-period EMA in the MACD). Being an oscillator the MACD-histogram fluctuates around the zero line.
Traders use the MACD-histogram to be alerted of imminent signal line crossovers in the MACD. The MACD is based on moving averages and moving averages lag price. The MACD-histogram does not have this slight lag. Hence a cross of the zero line by the MACD-histogram precedes an imminent MACD signal.
This example shows both the MACD (classic settings 26, 12,9) and the MACD-Histogram. The vertical dotted lines are to indicate that the cross on the MACD-histogram happens slightly earlier than the MACD crossover.