Balanced audio transmits a signal over three conductors: a ground (shield) and two signal conductors carrying the audio at equal amplitude but opposite polarity. At the receiving end, one signal is phase-inverted and added to the other. Any noise or interference that was induced equally on both conductors cancels out — this is called common-mode rejection.
XLR cables are balanced. Professional audio equipment — studio microphones, interfaces, mixing consoles, and powered monitors — use balanced connections specifically to allow long cable runs without noise pickup.
TRS (Tip-Ring-Sleeve) cables can also carry balanced audio. A balanced TRS cable has the same signal wiring as an XLR — tip is positive, ring is negative, sleeve is ground. Many audio interface outputs and studio monitor inputs use balanced TRS for shorter runs within a studio.
Unbalanced connections (RCA, TS mono cables, unbalanced 3.5mm) carry only one signal conductor and a ground. They work fine for short runs (under 15 feet) but become susceptible to noise, hum, and interference over longer distances.