Hinge offset is the distance from the face of the gate stile out to the hinge-pin centre line. With zero offset the gate pivots on its own face. Increase the offset and the pin sits further out, letting the gate swing past a wide trim, post wrap, or rebate without binding.
Offset matters most on rebated or 'set-in' gates where the leaf sits inside the post line when closed. A 25 to 40 mm offset is typical so the gate can swing 90 degrees fully clear of the rebate. On flush-mount gates it can sit at 10 mm or less.
CAD60 surfaces hinge offset on the per-post controls so the side-elevation and plan-view dimension lines reflect the chosen offset, not a default.