Basically this is a new implementation of large file support for Git, similar to git-media, git-annex or git-fat in the past. At a glance, this implementation and specification looks better than any previous implementation. Importantly this implementation is that it has no external dependencies on client binaries because it's written in Go, which is likely to lead to quick adoption. In contrast, git-media requires Ruby, git-fat requires Python and git-annex uses symlinks, which makes all 3 implementations difficult or impossible to use on Windows.
Another nicety of this implementation is that providing the server has the right endpoints, it requires no URL configuration for the large file storage. For HTTPS URLs, it appends "/info/lfs" to the remote URL and for SSH URLs it calls "git-lfs-authenticate" to determine authentication information (this allows the SSH authentication to provide an OAuth token which can then be used to download files over HTTPS from the same system). More details on this implementation are in the api.md reference document.
Important documents for implementation:
As someone who works in games development, this would be extremely useful in Phabricator, as large files often accumulate quickly and drastically increase the size of repositories.