I COULDN'T UNDERSTAND DRAG AND DROP API . Please tell me its use ?

Answer 1

HTML Drag and Drop interfaces enable applications to use drag and drop features in Firefox and other browsers. For example, with these features, the user can select draggable elements with a mouse, drag the elements to a droppable element, and drop those elements by releasing the mouse button. A translucent representation of the draggable elements follows the mouse pointer during the drag operation.
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Answer 2

I recommend you to learn JacaScript and then reviewing the API