Dynamic exit simply referring to exits that will adjust itself based on the asset you are trading.
The contrary is a fixed value SL, i.e. "I'll exit when it hit my 20pips SL and my Take Profit is 30 pips..."
Because the volatility for each asset is different on various timeframes, and market conditions.
Using the same rigid SL imposed a huge limitation on your strategy robustness.
A good example of dynamic exit is the Average True Range (ATR), as it will adjust the size of your stop-loss based on the volatility of the asset you're trading.