Regular divergence compares extremes at the ends of a move and suggests exhaustion. Hidden divergence compares pullback points inside a trend: price holds up better than momentum did, which is read as underlying strength.
In an uptrend the pattern is a higher low in price against a lower low in rsi; in a downtrend, a lower high in price against a higher high in the oscillator. It is used as a pullback entry filter rather than as a reversal call.
Be aware of how easy it is to find. With several oscillators, several lookbacks and freedom to choose which swing points to compare, some form of divergence exists almost always. Fix the oscillator, the setting and the swing definition in advance, or you are just drawing lines to fit a conclusion.
Related: divergence, rsi, pullback, macd-histogram, overfitting