macd itself is the difference between a 12 and 26 period exponential-moving-average. The signal line smooths that difference again, so a crossover means the momentum difference has turned relative to its own recent average.
Crossovers above zero are conventionally treated as stronger than those below, because they occur while the longer-term trend is still positive.
This is a third-order lag: an average of an average of an average of price. In ranges it flips constantly and produces whipsaw; in trends it can cross several times during a single healthy pullback. Most traders who use it profitably do so as a filter combined with structure, not as a standalone trigger.
Related: macd, macd-histogram, macd-zero-line-cross, signal-line, indicator-lag