The RSI is a popular momentum oscillator used by traders and investors to measure the speed and change of price movements in a stock. RSI oscillates between 0 and 100 and is considered overbought when it moves above 70 and oversold when it falls below 30. RSI is calculated using the average gain and average loss over a specified period, typically 14 days. The formula is a complex one, but in essence, it quantifies the magnitude of recent price changes, providing insights into a stock's overbought or oversold conditions.