If your image files are already stored on your hard disk, you just use the add to library button in the import module and select the folder you want to edit. Then you will see the thumbnail images in the lighttable view. (If the directory is large, it may take some time to generate the cache with the thumbnails.)
As already said the ‘add to library’ doesn’t duplicate images or move them it just lets DT know where they are. However, the ‘copy and import’ option can be a great feature to copy images from your camera’s SD card and organize them in the way you want to on your computer. Again the manual covers this well.