ASRC (asynchronous sample rate converter)
A converter that takes in digital audio on one clock and hands it out on another, with no requirement that the two clocks have anything to do with each other.
Two devices that both run at "48 kHz" never do so exactly. Hook an external source up over S/PDIF or AES3 to a system with its own clock and they drift apart; sooner or later a sample has to be duplicated or dropped — a click. The classic answer is to lock everything to a shared word clock. The other answer is an ASRC.
The circuit continuously measures the ratio between input and output clocks and interpolates new samples exactly where the local clock wants them. Off-the-shelf chips like the Analog Devices AD1896 made it cheap around 2000. Plenty of DACs use the same trick to insulate themselves from incoming jitter, at the price that every sample leaving them has been recalculated.