A stanchion is a post or rod used to support something. Stanchions hold things up.
You can almost see the word stand in stanchion, and that should help you remember what it means. A stanchion stands straight up, and it supports a larger structure. In just about any building, you will see posts going from the floor to the ceiling: those are stanchions. Stanchions are part of many walls too. If you took the stanchions out, the building might fall down. Stanchions always go up and down; they're vertical and provide support.