The histogram is positive when MACD is above its signal line and negative below. Because it measures the gap, it shrinks before a crossover happens, which is why traders watch it for early warning.
Its most cited use is divergence: price making a higher high while the histogram makes a lower high suggests the second push had less momentum behind it. That observation is real, but divergences can persist through several further highs before anything happens.
It is still derived entirely from lagging averages, so the early warning is relative, not absolute. Treat a shrinking histogram as a reason to manage an existing position rather than to reverse into a new one.
Related: macd, macd-signal-line, divergence, hidden-divergence, momentum-indicator