An RCA connector is a single pin surrounded by a ring (the signal conductor inside, the ground shield outside). It's unbalanced (see balanced vs. unbalanced), which means it carries the audio on one wire with no built-in noise cancellation. Simple to make, cheap to manufacture, and perfectly fine for short cable runs between gear that's sitting right next to each other.
The fun history part
The name comes from the Radio Corporation of America, which introduced the connector in the 1940s for an entirely different job: hooking a phonograph player up to the input of a radio's amplifier, so people could play records through their existing radio set instead of buying a whole separate record player with built-in speakers. It was meant to be a cheap, simple connector for a one-off internal job, not a long-term industry standard. Eighty-plus years later, it's still the default connector on turntables and home audio gear worldwide, which is a wild legacy for something RCA basically invented as a shortcut.
Fun side effect of that phonograph origin: RCA jacks are still sometimes called "phono connectors," which has nothing to do with phone calls and everything to do with the word "phonograph."
Where you'll find it in a DJ rig
- Turntables (almost universally, since the connector predates DJ culture by decades).
- Phono inputs on mixers, matched to the turntable's RCA output.
- Consumer-grade controllers and home stereo gear, where balanced connections would be overkill for the typical setup.
In the context of Headroom
RCA comes up in Module 2: Signal Flow right alongside balanced vs. unbalanced wiring, since knowing which connector you're holding tells you immediately what kind of signal path you're dealing with (and how much cable length you can get away with before noise becomes a problem).