I'm not quite sure how to describe this -- gesturing is so hard with a keyboard...
I believe it's a fixture that is attached to a door or gate (typically one at the top and bottom on the edge that swings open) that allows a vertical rod to be slid through.
The vertical rod then enters a hole in the bottom frame (or, rarely, the top frame if the rod is hinged with a twist handle) and thus secures the door/gate. To open the gate, you simply raise the rod by its handle and push or pull the door/gate open.
In the fully open position, people sometimes add receiving holes for the vertical rod that allows the gate/door to be held open. I've seen these on some barn doors too.