Drag and drop is like picking up toys from the floor and putting them in the toy box. You can move things from one place to another place on the computer just by using your mouse or finger if you're on a touchscreen device.
For example, if you want to move a photo from your desktop to a folder, you can click on the photo with your mouse, hold down the left button and drag it to the folder. Then you can release the left button and the photo will be dropped into the folder.
It's just like moving things around in real life, but on a computer screen. Dragging and dropping helps save time and makes it easy to organize and rearrange things on your computer.