A period is the length of the calculation window. An RSI with a period of 14 looks at the last fourteen candles, a moving average with a period of 200 averages two hundred. All the behavior of the indicator stems from this number.
The relationship is simple and unavoidable. A short period responds quickly but produces more false triggers. A long period creates a smoother picture but later signals. There is no setting that is both fast and error-free; this is an inherent tradeoff, not a configuration problem with a perfect solution.
Standard values like 14 for RSI or 12 and 26 for MACD were chosen decades ago for completely different markets and different timeframes. They are convenient as a starting point and as a common language, but they are not sacred. Meaningful selection depends on the asset and the timeframe: on a calm instrument and on a higher chart, the period can usually be increased, on a fast one - vice versa.
Here lies a classic mistake - selecting a period based on history to fit a perfect picture. A setting polished for a specific stretch of the past almost always describes exactly that period. A reasonable sign of stability is this: when the period changes slightly, the system's behavior changes smoothly, rather than collapsing. If the result relies only on one exact value, it's overfitting.
