Bearish divergence is price making a higher high while an oscillator such as RSI makes a lower high. Bullish divergence is the mirror image: a lower low in price that the indicator does not confirm.
What it describes is participation thinning out. Each new extreme is being reached by fewer buyers, or fewer sellers, than the one before, which is a plausible precondition for a reversal without being a schedule for one.
Divergence is a warning, not a trigger. It can persist through several new highs before anything happens, and the cemetery of short positions opened on the first divergence in a strong bull market is very large. Wait for price itself to break structure before acting.