The average of the last, say, 50 or 200 closing prices – it smooths the price line and shows the trend.
Many investors see the 200-day line as the border between long-term uptrend and downtrend. A 50-day average crossing above the 200-day is called a “golden cross”, crossing below a “death cross”. Simple trend rules based on moving averages have historically helped mainly by partly avoiding big downturns.