When you come off the throttle on a turbocharged car the built up charge of pressurized air has to go somewhere. If it can't, or if the air isn't released quickly enough then the turbo will slow down or even stall leading to a delay getting back on boost when you get back on the throttle.
This is where a dump valve comes in. It releases the built up pressure. Most cars have a dump valve of some sort fitted from the factory. This will have been tuned for the factory tune and boost levels though. On many cars a soft flexible diaphragm is part of the valve and these wear out. Once the valve is leaking it can cause problems with how the engine function