The panel Element Properties shows the editable properties of the element selected in the Elements panel.
The Bin tab is a special storage area that holds the elements you have deleted by right-clicking on the elements and selecting Cut, or by simply dragging the element to the Bin.
You may later drag them from this area and back into the form. This functionalty can be used to move an element between pages: drag the element to the bin, go to the page you want and drag the element from the bin to this page.
The Bin is always empty when you open the form in the Designer. When the Bin icon displays a number, the number shows how many elements it contains, i.e. how many elements you have deleted during this Designer session. When you close the form the bin will be emptied, so it is only a temporary hold area.