Syncing is simply keeping a collection of files on your I-pad/Lightroom Mobile, in sync with the desktop versions - your machine to your machine, across the internet: I suppose this would be via Lr's servers, but the idea is simply to give you mobility, and the ability to use Lr away from your desktop for a given session or shoot - or to wirelessly move "Smart Preview" versions of a collection, to your mobile device.
I don't use an I-Pad/Pod/Phone or Lr Mobile, but that's the basic premise: it's not really intended to be a way to have mobile access to your entire accumulation of images - especially when we're getting into multiple tb's worth of images: besides, your I-Pad hasn't got the capacity for that.
That said, I suppose with a small "portfolio" it'd be doable.
Anyway, pretty good explanation here:
"In order to use LRM, you login with LRD and enable sync, then set specific Collections to sync. It automatically generates the necessary raw proxy images, though it doesn't indicate within LRD that there's a Smart Preview in the usual spot beneath the histogram. As long as you're connected to a network, the selected Collections and any operations you perform on the images within are automatically synced between the two. If you want to be able to work on images while disconnected from the network, you have to enable offline editing; otherwise, it dynamically downloads each image as you open it and caches it temporarily
The images are small enough that you don't have to wait inordinately long for them to load, but as a group they're big enough that they can take up a lot of space on a limited-capacity iPad. (Pity that amazing compression algorithm from HBO's "Silicon Valley" doesn't really exist.) You'll probably want to come up with a syncing strategy to manage your storage. The syncing itself happens pretty quickly and seamlessly, at least over Wi-Fi, though there is a noticeable pause before the interface becomes available as you wait for the proxy to download when working online. And you can work on already-downloaded images while it continues to sync.
You can only work with a single catalog at a time. (As a refresher, Lightroom images are mapped in databases called catalogs, in which which you can create collections, and Lightroom only supports a single open catalog.) However, the app will attempt to sync with whatever open catalog you're logged into and will offer to switch to that catalog. So, for instance, if you're working with the images from a machine at the office, then go home and log on with a different machine, you can switch to syncing with that catalog. To do so wipes the previously synced catalog from your device. It can also import photos from your camera roll and automatically sync them back to the desktop".
Lightroom Mobile is an in-house alternative to Photosmith, really - so what applies to Photosmith, in terms of purpose and usage seems broadly to apply to Lr Mobile (Photosmith being more feature rich).