Patmos was where John was imprisoned when he received the vision recorded in the book of Revelation.
I, John, your brother and companion in the suffering and kingdom and patient endurance that are ours in Jesus, was on the island of Patmos because of the word of God and the testimony of Jesus. (Rev 1:9)
Patmos is a small Greek island in the Aegean Sea, and is marked on the map below.
For more information, see the Wikipedia article on Patmos.