This describes it well.
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In synchronous USB D/A converter clock comes from the
computer. Usually D/A converter runs from internal clock that is synchronized with incoming USB clock by PLL (Phase Lock Loop) but it is less than perfect. This scheme is used in pretty much every CDP, except here incoming USB clock can be very jittery.
Asynchronous USB uses fixed internal DAC's clock for the D/A converter. This clock is independent (asynchronous) to USB data rate. For that reason data has to be buffered. DAC has to request different number of samples each frame to keep data buffer between underflow and overflow.
Because in asynchronous USB DAC D/A converter is fed from the data stored in a buffer it operates from stable internal clock reducing jitter.