The histogram converts the distance between the lines into a visual bar height. In MACD, for example, the bar shows how far the main line has moved away from the signal line. Bars above zero mean the main line is above, below zero - below, and crossing zero coincides with the moment the lines intersect.
It is more interesting to read a histogram by its dynamics rather than by its sign. Rising bars indicate that the lines are diverging and the movement is accelerating. Decreasing bars mean that the lines are converging and the movement is losing momentum, even if the price is still moving in the same direction. This feature often provides an earlier warning than the crossover itself.
From here also comes the divergence on the histogram: the price draws a new extremum, but the height of the bars does not correspond to it. The divergence indicates that there is less strength behind the new extremum than behind the previous one.
The limitations are typical for derived values. The histogram is calculated from lines, which themselves are calculated from past prices, so the lag doesn't go away—it just appears differently. And on small timeframes, the bars constantly change height within an unfinished candle, so conclusions based on a 'nearly formed' histogram often have to be reversed after a couple of minutes.
