A hidden tang is a narrow tang, often a rod or a stick, that runs inside a one-piece handle rather than being sandwiched between two scales. No steel shows anywhere on the grip. It is the traditional construction for stacked-leather and turned-wood handles.
The handle is drilled or slotted, the tang slides in, and the whole thing is glued, pinned or drawn tight by a threaded pommel nut at the butt. Guard and pommel deals with that end. Because nothing is visible, the handle can be a single unbroken piece of wood, horn, stacked leather washers or molded polymer.
A hidden tang can be nearly full length and still be hidden. Hidden describes visibility, not extent, which is why it is not a synonym for partial tang even though the two often overlap.
The look is the point. A hidden tang gives a clean, seamless handle with no scale line and no exposed metal to conduct cold or catch on a sheath. It is why so many classic hunting knives and puukko-style blades use it.
Strength depends entirely on how well the tang is secured. A threaded pommel that draws the handle tight is a genuinely strong assembly. A tang held only by adhesive is the one to be careful with in a heavy-use knife.