The relative strength index (RSI) is a technical analysis indicator that measures the strength of price movements to help identify overbought and oversold conditions in an asset. It oscillates between 0-100, with readings above 70 indicating an overbought asset and below 30 indicating an oversold asset. Divergences between the RSI and price trends can signal trend reversals. The RSI is used to help identify trading opportunities by looking for divergences, chart patterns, and signals at overbought and oversold levels, though it does not guarantee profitable trades and must be used carefully along with other indicators and analysis.