When placing elements, for example during a drag-and-drop operation, a drop guide will appear to help you place it correctly.
When hovering an element, you may be able to place it before / in / after the target. The drop guide will highlight it accordingly:
Sometimes you will find an empty placeholder where you can can drop new elements. The drop guide will highlight it like this:
Valid drop targets will be highlighted with blue, invalid drop targets highlighted with red: