thumbs.db is a hidden Windows system file. It's a database for thumbnail cache used by system, so it can change over time (according to
https://en.wikipedia.org/wiki/Thumbs.db). In references section there's a link to Microsoft Support page describing how to turn off this caching, but I guess thumbs.db files already there will stay. It's safe to delete the file, usually I deleted it just before creating a new torrent. It would be nice if Tixati had some option to skip system or hidden files on Create New Seed window.