A feature of Windows (and other graphical operating systems including Apple System 7, IBM OS/2 and Unix/X). The system means you can move a highlighted icon or piece of text. For example, if you want to delete a file from the Windows 95 desktop, you move the pointer to the file's icon, click once to highlight the icon, and then press and hold down the left-hand mouse button. You have now picked up the icon and can move it around the desktop. With the mouse button still pressed down, move it on top of the Recycle Bin icon. The Recycle Bin icon will change colour to indicate that it's recognised you want to use it. Now release the mouse button and you've deleted the file using drag and drop.
In other applications, for example Word for Windows or other word processors, you can highlight a section of text in a letter and then click and hold down the left-hand mouse button to move the section of text to another place in the document, or even to another document.