1/4” (6.5mm) is a larger version of the same connector family as 3.5mm, commonly found as a headphone output or line-level input on DJ controllers, mixers, and speakers. Like its smaller counterpart, it’s wired as either TRS (stereo/balanced) or TS (mono/unbalanced).
Controllers that natively output 3.5mm or RCA often need an adapter up to 6.5mm to match a venue mixer’s input.