Arbaeen is the 40th day after Ashura in which Imam Hussein (as) and his companions martyred. In Arbaein, due to some historical documents, Imam Sajjad (AS), Hazrat Zeinab (the sister of Imam Hussein), and some other relatives of the Imam, came back to Karbala and mourned for Imam and other martyrs of the Ashura.
After Saddam, walking to Karbala was released. Since then, every year, millions of Shia Muslims from all over the world travel to Karbala, the holy city of Iraq, to commemorate the religious ceremony of Arbaeen, the 40th day after the martyrdom of the third Shia Imam.
The walk from Najaf to Karbala is one of the most enlightening acts of worship a pilgrim can perform. The suffering and hardship imposed on the family of Imam Hussain (as) and the grandchildren of Prophet Muhammad (s) is felt in every step, and thus the love and respect for this holy family increases in the heart of the believer with every step that is taken towards the holy land of Karbala.
The following video from AhlulBayt TV is an interview with people dealing with the event of Arbaeen 2013 in Iraq: (Link in Youtube)