The MACD line itself
The MACD line is the difference between two exponential moving averages (EMAs) of price, typically the 12-day EMA minus the 26-day EMA. An EMA weights recent prices more heavily than older ones, so it reacts faster to a change in direction than a simple average would. When the 12-day EMA sits above the 26-day EMA, the MACD line is positive, and that tells you short-term momentum is running above the medium-term trend. When it flips negative, the reverse is true.
This line is the raw material. On its own it tells you the gap between two speeds of price movement is widening or narrowing. It does not, by itself, tell you when to act.
The signal line
The signal line is a 9-day EMA of the MACD line. It smooths the MACD line further, and it is the line used to define crossovers: MACD crossing above the signal line is conventionally read as a bullish signal, crossing below as bearish.
Here is the problem with leaning on that crossover as the main event. The signal line is a moving average of a moving average of a moving average of price. Every layer of smoothing adds delay. Because of that smoothing, the MACD line can cross the signal line only after a real move in the underlying share has already been running for several sessions. On a share that can move on company news or a currency shift well before a crossover confirms, a trader waiting for that cross is trading yesterday's momentum.
The histogram
The histogram is the MACD line minus the signal line, plotted as bars around a zero line. This is the part I think does the actual work, and it is easy to treat as decoration.
Because the histogram measures the gap between MACD and signal, it starts shrinking before the two lines actually meet and cross. A shrinking histogram, even while the bars are still above zero, tells you the MACD line is losing speed relative to its own smoothed average. That is the momentum shift showing up early. The crossover, when it finally comes, is just the moment the histogram touches zero. By then the more useful information, the deceleration, has already been visible for several bars.
Work through a simplified version with round numbers. Say a share's 12-day EMA sits at ₦58.40 and its 26-day EMA at ₦55.10, giving a MACD line value of 3.30. If the 9-day signal line reads 2.10, the histogram is 1.20. If, over the next four sessions, the MACD line eases to 3.00, 2.70, 2.50 and 2.30 while the signal line drifts more slowly to 2.20, 2.28, 2.34 and 2.32, the histogram falls from 1.20 to 0.80, 0.42, 0.16 and, on the fourth day, minus 0.02. The crossover only registers on that fourth day. But the histogram had already told you momentum was fading from day one of that run. That gap between "momentum is fading" and "the lines have crossed" is the whole argument for watching the bars over the line.
Where this leaves the MACD line
The MACD line still matters as context. Its position relative to the zero line tells you whether the short-term trend sits above or below the medium-term one, which is a different question from whether momentum is speeding up or slowing down. A rising histogram while the MACD line sits below zero is a different situation to a rising histogram above zero, and it is easy to conflate the two. But as a standalone trigger, the MACD line is the slowest of the three parts to reflect a shift in the underlying price, since it is itself an average of two other averages.
What the indicator does not do
MACD is built entirely from price, specifically from EMAs of the closing price. It carries no information about volume, no information about why a share is moving (a dividend announcement, a regulatory notice, a currency shift), and no information about valuation. Anyone reading MACD in isolation, without checking volume or the news behind a move, is reading half the picture.
The histogram is the part that earns its place on the chart. It is not a separate indicator bolted on for visual effect, it is the difference that reveals a change in speed before the crossover makes that change official. Treat the signal-line cross as confirmation of something the histogram already showed you, not as the first sign of it.