RSI above 70 is conventionally called overbought and below 30 oversold. In a strong trend it can sit above 70 for weeks, so those labels are descriptions, not signals.
Its more useful application is divergence: price makes a new high while RSI makes a lower high, which suggests momentum is fading.
Example: a stock rallies from $50 to $70 with RSI at 82. It pulls back to $65, then rallies to $72 with RSI at 74. That lower RSI high on a higher price high is bearish divergence.
Related: divergence, macd, mean-reversion, trend