Read Tombstone (typography) (Wikipedia)

In mathematics, the tombstone, halmos, end-of-proof, or Q.E.D. symbol "∎" (or "□") is a symbol used to denote the end of a proof, in place of the traditional abbreviation "Q.E.D." for the Latin phrase "quod erat demonstrandum", meaning "which was to be demonstrated". In magazines, it is one of the various symbols used to indicate the end of an article. In Unicode, it is represented as character U+220E ∎ END OF PROOF (HTML ∎). Its graphic form varies, as it may be a hollow or filled rectangle or square.