In addition to aggregating articles about books, music, TV series and movies, Founce tries to figure out who and what are mentioned in those aggregated articles. A mention is when Founce is able to establish that a person, band or creative work was included in an article. Essentially, it means that a person, band or creative work was mentioned in an article.
Distilled articles shown in the feeds include a mention count in the top right hand corner. It's an italicized "Mentions" label above a number. The number represents the total number of people, bands, books, albums, TV series and movies found within the article.