The three components
The MACD line is the fast average minus the slow one, conventionally twelve and twenty-six periods. The signal line is a nine-period average of that line. The histogram is the gap between them.
When the fast average pulls away from the slow one, the line extends; when they converge, it returns toward zero.
What a crossover indicates
The MACD line crossing its signal line means the recent rate of change has shifted relative to its own recent average. It is a statement about momentum having already turned.
Because both inputs are smoothed averages, the turn happened before the crossover printed. The indicator is late by construction, and shortening the periods trades lag for false signals.
Where it misleads
In a sideways market it crosses constantly, producing a stream of signals that each lose a little to costs. In a strong trend it can stay extended for a long time, which reads as overextension and is simply a trend.
The histogram is often read as acceleration, which is reasonable, but it is an average of averages and should not be treated as precise.
Reasonable use
As a smoothed view of whether momentum is building or fading over your chosen horizon, alongside depth, volume and the actual price level. Not as a trigger on its own.
FBT Swap shows MACD among other readings with its parameters stated, and shows an unavailable state rather than an invented value when the source is down.