What about MACD?

As it's name implies MACD (Moving Average Convergence & Divergence) is another indicator based on the EMA. It is best to review it in 3 parts:
1) MACD line (blue)
2) Signal line (red)
3) MACD Histogram (the dark vertical bars)

The default MACD has 3 parameters i.e. MACD (12, 26, 9).
The first 2 parameters are for the short (fast) and long (slow) EMAs.

1) What does the MACD line signifies?
It is the difference of the fast EMA and the slow EMA. When positive and increasing, it means the fast EMA Diverges from the slow EMA, it's magnitude shows the price acceleration comparison between the 2 EMAs. When it reaches a peak, price acceleration paused before the reversal and Convergence of the EMAs.

In other words, the gradient of the MACD line signifies the price acceleration. Mathematically, it is equivalent to the 1st derivative of "rate of price change" or the 2nd derivative of (price vs time) function.

2) The signal line is a EMA(9) of MACD line, what does this mean? Remember the filter properties of MA? It is the smoothened MACD line that introduce a lag and reduced peak-peak values. It is a time lag signal to generate crossovers (similar to MA crossings) to indicate change of direction.

3) The MACD histogram is the plot of difference between the MACD & the Signal line. It brings the cross over to the zero line. If the decision criteria is based on the crossovers, then histogram above zero signifies an up trend and histogram below means a down trend.

A more subtle point is it provides a visual indication to price acceleration. Note it always peaks before the MACD line? It's gradient shows the 2nd order (pseudo?) derivative of "rate of price changes" or the 3rd derivative of (price vs time) function. In simple term it reveals the "acceleration within an acceleration".

Hitting the peak is the first sign of 'possible reversal' but do not guarantees it! As it is evident graphically there can be higher subsequent peaks.

'Differences of MA' is used to perform 'derivative' function on time series data equivalent to 'differentiation' operation as in calculus! Clever, isn't it?

Varying the first 2 MACD parameters allow tracking of short or longer term trends. While the 3rd parameter merely add a further delay for trend reversal indication.

With these understandings, one may appreciate its inner workings better & choice of the parameters values used in MACD.