What is meant by the words of the Prophet (blessings and peace of Allah be upon him), Hajj is Arafah is that in Hajj it is essential to stand in Arafah. Whoever does not stand in Arafah has missed out on Hajj. It does not mean that the one who stands in Arafah no longer has to do any of the actions of Hajj, according to scholarly consensus, because if a person stands in Arafah, he still has to do some actions of Hajj, such as staying overnight in Muzdalifah, tawaaf al-ifaadah, sai between as-Safa and al-Marwah, stoning the Jamaraat and staying overnight in Mina. Rather what is meant is that standing in Arafah is an essential part of Hajj and if he does not stand in Arafah then there is no Hajj for him. Hence the scholars said: Whoever misses the standing (in Arafah) has missed Hajj. End quote.
Reproduced from Islam QA