Without knowing the context surrounding those sentences there is only so much to offer to explain it, and it states what it means fairly clearly if you know what runtime loading is.
Runtime loadng is when you load images dynamically from external locations. You could have folder of images that are used by your swf file. If you should want to change the images that the file uses, you only have to replace them in the folder that it loads them from. If you had designed those images directly into the file instead of loading them dynamically you would have to edit the file and create a new swf every time you want to change the images