The measures the distance between two EMAs: MACD line = EMA 12 minus EMA 26 of the closes. The is an EMA 9 of this MACD line, the shows the distance between the two. 12/26/9 is the usual standard, not a law of nature. Like every average, the MACD lags behind the price.
The zero line
Above zero, the EMA 12 is above the EMA 26: the more recent prices are higher on average. A move across the zero line is nothing other than a crossover of these two EMAs – so it lags too.
Signal line and histogram
If the MACD line rises above the signal line, the histogram turns positive; if it falls below, negative. If the bars shrink, the MACD line is approaching its own average – the pace of the move is slowing. That is not a forecast, and in sideways phases there are many crossovers without consequence.
Do the math
EMA 12 = 105.40 and EMA 26 = 103.90 give a MACD line of 1.50. If the signal line is at 1.20, the histogram is 1.50 − 1.20 = 0.30. The signal line only exists once there are 9 MACD values – in our chart with 12/26/9 from the 34th candle; other programs start differently.