The «SQL expression» is added to WHERE clause of view feed query. You can match on ttrss_entries table fields and even use subselect to query additional information. This functionality is considered to be advanced and requires basic understanding of SQL.
Match all unread articles:
unread = true
Matches all articles which mention Linux in the title:
ttrss_entries.title like '%Linux%'
See the database schema included in the distribution package for gruesome details.