AUX (3.5mm) is the small, common consumer-size audio jack used for headphones, laptop headphone/output jacks, and many compact or portable DJ controllers. It’s most often wired as TRS (stereo), though some mono TS connections exist.
It’s the connector you’ll run into most for connecting headphones to controllers (as well as laptop-only setups). Adapters can easily step up this connector to 1/4” (6.5mm) or RCA for larger mixers and venue equipment.