Arda Marred (Arda Hastaina in Quenya) is a term referring to the world as it now is, stained by the evil of Melkor. The Discords of Melkor in the Music of the Ainur, and especially his later actions upon Earth, have caused evil and suffering that were not part of the original plan of Eä, according to the Elves. Melkor's very essence seeped into the fabric of Arda, so that nothing in it, and nothing that is sustained by it, is free from the Marring. This is why the Free Peoples of Arda can be deceived and enticed to evil.
There are rumors that Eru will come to the Arda and amend the Marring himself, and that a new world will one day be made: Arda Healed (Arda Envinyanta); but no one in Arda, possibly not even the Valar, knows this for sure.