A !SYNC file is a partially synchronized file created by BitTorrent, a P2P file synchronization tool. It serves as a temporary placeholder for any file that was not downloaded and synced.
The !SYNC file extension is used to mark temporary files that are currently being downloaded or that are left partially received because of an interrupted synchronization. The extension is removed once the file is fully downloaded. If you remove files with the !SYNC extension before they are completed, BitTorrent Sync begins to transfer the file again.