The pin/unpin button is always visible in the widget menu. Click it to get the dropdown options.
You may have created a scrolling page and you want to have a widget float over the page and stay in view while you scroll the page in preview mode. Use the pinned/unpinned option to control what moves with the page.
Any widget can be fixed on top of a long/scrolling page (‘pinned’) or allowed to move with the scrolled page (‘unpinned’).