.sync/StreamsList - A user editable file allowing you to "white list" alternate streams, xattrs and resource forks.
.sync/IgnoreList - A user editable file allowing you to "exclude" certain files/sub folders from being synced. To learn how you can edit IgnoreList, click here.
.sync/Archive - Resilio Sync, by default, won't actually delete any of your files/folders. If a corresponding file/folder is deleted on another device, it will simply be "moved" into the .Archive folder on all other devices, rather than being permanently deleted. Click here to learn more.
Note that the above mentioned files are hidden by default. To view these files, you should enable ’Show hidden files and folders’ option in your OS.