Above 70 is called "overbought" and below 30 "oversold". In strong trends RSI can stay overbought or oversold for a long time, so it is a warning, not a sell or buy signal on its own.
Trading
RSI (Relative Strength Index)
Also called: RSI · overbought · oversold
In shortA momentum indicator from 0 to 100 that shows whether an asset has risen or fallen unusually fast.