As an English phrase, it denotes an object or topic which sows dissension and anger.
An apple of discord is a reference to the Golden Apple of Discord which, according to Greek mythology, the goddess Eris inscribed “to the fairest” and tossed the apple in the midst of the festivities at the wedding of Peleus and Thetis, thus sparking a vanity-fuelled dispute between the goddesses Hera, Athena and Aphrodite which eventually led to the Trojan War.
Kallistēi is the word of the Ancient Greek language inscribed by Eris on the Golden Apple of Discord. In Greek, the word is kallistēi (the dative singular of the feminine superlative of kalos, meaning ‘beautiful’). Its meaning can be rendered “to the fairest one”.
On a trivial note: in the Eixample district of Barcelona is a block nicknamed in Spanish: La manzana de la discordia (L’illa de la discòrdia in Catalan), the reason for this usage is because manzana means both ‘apple’ and ‘city block’ in Spanish. It was named ‘block of discord’ because the buildings in the district feature three different interpretations of modernist architecture by the artists: Gaudí, Domènech i Montaner, and Puig i Cadafalch.