RSI averages the up closes and the down closes over a window, conventionally fourteen periods, and expresses the result as a number between zero and one hundred. Readings above seventy are called overbought and below thirty oversold.
Those labels mislead more than they help. In a strong uptrend RSI can hold above seventy for weeks while price keeps rising, so selling on the first overbought print is a reliable way to exit early in exactly the moves worth holding.
Its more useful reading is divergence: price making a new high while RSI makes a lower high says the momentum behind the move is fading, even though the trend is intact. That is a warning to tighten risk, not an instruction to reverse.