The MACD line is the 12-period exponential moving average minus the 26-period one. The signal line is a 9-period average of that, and the histogram is the difference between the two. A widening histogram means momentum is accelerating.
The common reading is the crossover: MACD above its signal line for a bullish turn, below it for a bearish one. Because all of it is built from averages, the crossover arrives after the turn in price, not before it.
It works best where it was designed to work, in trending markets on daily or longer charts. In a sideways range it produces a stream of crossovers in both directions that cost money to follow.