MACD tracks the distance between two moving-averages. When the fast average pulls away from the slow one, momentum is increasing; when they converge, it is fading. The histogram shows the MACD line minus its signal line.
Traders use crossovers of the MACD and signal line and divergence against price. Like RSI it lags, and in a range it produces many whipsaws.
Example: with the 12 EMA at $101.50 and the 26 EMA at $100.00, MACD is +1.50. If the signal line is at +1.20, the histogram is +0.30 and rising.
Related: moving-average, rsi, divergence, whipsaw