It’s a hand tool with a U-shaped (open) jaw on one end only.
The open jaw grips two opposite faces of a nut or bolt head.
Because it's open, it can slip onto a fastener from the side — useful when you can’t place a closed (box) wrench over the top.
Typically made of strong tool steel (e.g. chrome-vanadium), sometimes drop forged, often with a corrosion-resistant finish.
The single-end design keeps one side free (no second head), possibly saving material or making the tool lighter or more specialized for a particular application.
