Moving average
A moving average calculates the average price over the last N periods and updates with every new candle. It smooths out noise and shows the trend.
The simple moving average (SMA) adds up the last N closes and divides by N. The exponential moving average (EMA) gives more weight to recent prices. The classic periods are 20, 50, 100 and 200.
Price above a rising average: bullish bias. Price below a falling average: bearish bias. The 200-day average is watched by many investors on indices such as the S&P 500.
Moving averages also act as dynamic support and resistance, and as crossover signals. Their weakness is lag: they confirm a trend, they don't predict it, and they throw out lots of false signals in a range.