A 50-day moving average (50 DMA) is the average closing price of the last 50 days, updated each day. It smooths out daily wiggles so the underlying trend is visible.
Two lines you'll see everywhere: 50 DMA (short-term trend) and 200 DMA (long-term trend). When price is above the 200 DMA, the stock is in a long-term uptrend by convention.
Golden Cross (50 DMA crosses above 200 DMA) is often described as bullish. Death Cross is the opposite. Both are noisy signals — they work sometimes and fail loudly other times.
Best use: as a sanity check on trend, not as a buy/sell trigger. Combine with fundamentals.