Relative Strength Index (RSI)
A momentum oscillator from 0 to 100 comparing the size of recent gains with recent losses.
In plain English
Readings above 70 and below 30 are conventionally called overbought and oversold, and a 14-period setting is the common default. In a strong trend RSI can stay “overbought” for a long time.
Example
RSI reading 78 during a steady uptrend is not, on its own, a reason to expect a fall.