Phantom power
DC power for the microphone down the same cable as the audio, invisible to anything that doesn't need it.
Condenser microphones need power to polarise the capsule and run their built-in preamps. Phantom power puts +48 V through resistors onto both signal conductors of a balanced XLR cable, referenced to the shield. Because the voltage is identical on both legs, the balanced input cancels it out, and a dynamic mic with a balanced output never notices. Neumann introduced the scheme in the 1960s; today it is specified in IEC 61938 as P48.
So the "48V" button on every audio interface is not optional for condensers. It is, however, bad news for old ribbon mics wired unbalanced and for miswired cables, and anyone who engages it with the fader up gets a loud pop through the monitors as a free lesson.