Bearish divergence: higher price high, lower indicator high. Bullish divergence: lower price low, higher indicator low. It is most often read on rsi or macd.
Divergence can persist for a long time in a strong trend and is a warning, not a trigger. Traders typically wait for a structural break, such as a lower low, before acting on it.
Example: a stock prints lows at $30 then $28, while RSI prints 25 then 34. The second low was made with less downside momentum, which is bullish divergence.