The RSI is a momentum indicator scaled 0 to 100 that measures how fast and how far price has moved recently. Above 70 is often called overbought; below 30, oversold.
Overbought does not mean 'sell now' — strong trends can stay overbought for a long time. Traders also watch RSI divergence, where price makes a new extreme but RSI does not, as a possible reversal clue.