RSI (Relative Strength Index) is a momentum oscillator, plotted on its own 0-to-100 scale below the price chart, that measures the speed and size of recent price changes. Readings above 70 are commonly watched as a sign an instrument may be overbought, while readings below 30 are commonly watched as a sign it may be oversold - though, like every indicator, this describes a pattern worth investigating further, not a guaranteed reversal signal.
Related Reading
See how RSI is calculated and read in RSI Explained on TradingView.