A 50-period simple moving average adds the last 50 closes and divides by 50. Each new bar drops the oldest value and adds the newest, which is why the line can move sharply when a large old value falls out of the window even if today's price barely changed.
It is the most widely watched smoothing in markets, and the 50, 100 and 200 period versions on daily charts are followed by enough participants that they occasionally behave like real levels through sheer attention. See moving-average-as-support.
Its defining property is indicator-lag: the line is by construction an average of the past, so it turns after price does, by roughly half the window length. No parameter choice removes this. Anyone claiming a moving average predicts anything has misunderstood the arithmetic.
Related: exponential-moving-average, moving-average, indicator-lag, moving-average-crossover, lookback-period