A moving average smooths a price series by plotting the average price over a rolling window — a 50-day moving average at any given point is the average closing price of the preceding 50 days. As each new day's price comes in, the oldest day drops off the window and the average shifts accordingly, so the line updates continuously rather than being recalculated from scratch.
The point is to filter out day-to-day noise and make the underlying trend easier to read. A shorter window (10 or 20 days) tracks the price closely and reacts fast; a longer one (100 or 200 days) is smoother and slower to turn, and is often used as a reference line for the "primary" trend.
A common technical signal is a crossover: a shorter moving average crossing above a longer one is often read as a bullish signal ("golden cross"), and crossing below as bearish ("death cross"). Like any single indicator, crossovers generate false signals regularly and are typically used alongside other evidence, not alone.