Drag and drop lets someone move an item by holding it, moving it, and releasing it somewhere else. It is like picking up a book and placing it on another shelf.
A person might reorder tasks in a list or move a file into a folder. The page should show where the item will land so the result is not a surprise.
Dragging can be hard on a touchscreen or for people who cannot use a mouse precisely. Offer another way, such as buttons to move an item up or down, when the action matters.