The MACD is built to catch the exact moment momentum changes gear. It does this by comparing two rolling averages of the price — one that reacts quickly to recent moves, and one that reacts slowly.
When the fast average crosses up through the slow one, it is a sign that upward momentum has just taken over — a possible turn from falling to rising. When it crosses back down through the slow one, downward momentum is taking the wheel instead. These moments are called crossovers, and they are what MACD is famous for.
In everyday terms, it is a way of noticing the tide beginning to turn early, rather than waiting until the change of direction is obvious to absolutely everyone — by which point much of the move may already be gone.