Labels are generated by using SQL expressions. 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 some understanding of SQL.
Match all unread articles:
unread = true
Matches all articles which mention Linux in the title:
ttrss_entries.title like '%Linux%'
Matches all articles for the last week (PostgreSQL):
updated > NOW() - INTERVAL '7 days'
See the database schema here or included in the distribution package for gruesome details. The relevant tables are ttrss_entries and ttrss_user_entries.
Feel free to post more clever examples of labels on Tiny Tiny RSS forums.