The Relative Strength Index (RSI) compares the size of recent gains to recent losses. It sits between 0 and 100.
Conventional reading: RSI above 70 is 'overbought' (price rose fast; may cool off). RSI below 30 is 'oversold' (price fell fast; may bounce).
In strong trends, RSI can stay above 70 or below 30 for weeks without any reversal. Using RSI as a standalone buy/sell signal usually disappoints.
Better use: as one signal among several. If a stock's fundamentals are strong AND RSI drops below 30 after a market panic, it's worth a closer look.