An alias is a pointer to a file or folder on the Mac, similar to a shortcut on Windows. To create an alias using drag and drop, simply hold down both Option and Command keys on the keyboard while dragging and dropping the file to the destination folder.
While dragging, the on-screen cursor will change to show the alias icon which is a bent arrow. Once an alias is created, you can see the bent arrow attached to the left-bottom corner of the alias file icon.
Since an alias file is just a pointer to the actual file or folder, deleting the alias file will not delete the actual file or folder.