RSI is a momentum oscillator on a 0-100 scale, built from the ratio of average gains to average losses over a lookback, classically 14 periods. Readings above 70 mark a stretched advance, below 30 a washed-out decline, with the middle band reading as neutral.
RSI's classic failure mode is trending markets: a strong uptrend can hold RSI above 70 for weeks, so the stretched reading marks strength as often as reversal. That is why composite scoring uses RSI as one bounded vote among several rather than a trigger, and why momentum alignment checks read RSI bands (for example 40-80 in breakout screens) rather than the extremes alone.