Packable bags are bags made of strong but very compactable materials. The purpose of a packable bag is that you can pack it in another bag when it's not in use. This is really helpful when you're traveling and want to carry only a suitcase, but then want to have a backpack or duffel bag when you're at your destination. Then, you just pull out your ultra-compactable backpack, load it with your goodies needed for that day, and get moving, and you didn't really affect your suitcase space by bringing the extra bag.
Zumiez is the place to get packable bags - including duffel bags and backpacks - from workwear and streetwear brands like Herschel Supply.