Smart Previews are low resolution versions of your images that you can build in order to edit your photos when they’re not available, such as when they’re stored on an external drive and you’re traveling without it. Lightroom can create smart previews as you import your images or you can build them for images that you’ve already imported and they’re stored in your Catalof. The Smart Previews are lossy DNG files, which is Adobe’s raw format, and they can be as big as 2540 pixels on the longest side. They take up suprisingly little space, making them very popular with travelers. Some people find them so handy they keep their Catalog on Dropbox so they always have it available making it even easier to work.