If set to true, then Nautilus will have a feature allowing you to delete a file immediately and in-place, instead of moving it to the trash. This feature can be dangerous, so use caution.
Speed tradeoff for when to show a preview of text file contents in the file's icon. If set to "always" then always show previews, even if the folder is on a remote server. If set to "local-only" then only show previews for local file systems. If set to "never" then never bother to read preview data.