What they are
A moving average is the average closing price over a set number of recent sessions, worked out again each day so the line moves with the price. A simple moving average weights every day the same. An exponential moving average gives recent days more weight, so it turns sooner.
The lengths traders use
The 20 or 21-day average covers about a month of trading, the 50-day about ten weeks, and the 200-day about ten months. Short averages follow the price closely, and long ones smooth out all but the largest moves. The 200-day is the line most often quoted as a stock's long-term trend.
How traders read them
A price above a rising average is in an uptrend on that timescale, and a price below a falling one is in a downtrend. Traders watch how a stock behaves when it pulls back to a widely followed average, and a close well through one on heavy volume is often read as a change in the trend. Averages lag the price by design: they describe a trend rather than call its turns.
Moving averages on Breakaway
Every public stock page shows the 50-day and 200-day averages, how far the price is from each, and the date of their last cross. Several of the 14 chart checks use moving averages: the stacked averages (MA FAN), the rising 21-day EMA, and the weekly trend. The Quality Score's long-term trend line reads the price against the 200-day exponential average, and whether that average is rising.