Word clock
A shared timing signal that makes several digital audio devices take their samples at exactly the same moment.
Two devices both running at 48 kHz still each have their own crystal, and crystals disagree slightly. Without a common clock the streams drift apart, and you get clicks whenever a buffer runs dry or overflows. The fix is to make one device master, sending a square wave at the sample rate over 75-ohm BNC cable, and have the others lock to it with a PLL.
How clean the clock is matters too. Variation in when each sample is taken — jitter, the same word as in networking but measured in picoseconds rather than milliseconds — turns into noise and distortion in the conversion. This has given rise to expensive standalone master clocks, whose benefit measurements have struggled to locate.