Like an average grade at school: one bad mark doesn't turn a top student into a failing one. The average likewise doesn't react to one or two odd candles.
A 20-candle average, at every point, takes the last 20 closing prices and calculates their average. With each new candle the window shifts — hence “moving”. In the simulator it's called SMA 20. There are also 50- and 200-candle averages: the bigger the number, the smoother the line and the larger the trend it shows.