Lightroom on your mobile device (I’m just going to call it Lightroom mobile from here on out, or even just LR mobile so it doesn’t get tedious) only syncs collections (not folders and not smart collections). So, to get your images over to LR mobile, you first need to put the photos you want in LR mobile in a collection. Then, click the box that appears to the left of the collection’s name in the Collections panel and a little sync icon appears (that icon is shown circled above in red). Of course, before any of this will work, you have to log in with your Adobe Creative Cloud ID in Lightroom on your desktop (which I’ll just call Lightroom desktop or LR desktop from here on out) by going to Lightroom’s preferences (Command-, [comma; PC: Ctrl-,]), clicking on the Lightroom Mobile tab, then clicking on the Sign In button. Once you’re logged-in, click on the Lightroom logo (in the top left of Lightroom) and, from the pop-up Activity Center that appears, click on Sync with Lightroom mobile (as seen above on the right).