It means any obligatory act on which the validity of Hajj depends and which is redeemed by offering a sacrifice.
According to this school: the obligatory act may be represented as the Essential [Al-Rukn], and it includes the Requisite [Al-Shart].
The Four Obligatory Acts of Hajj:
2. Standing by `Arafah
3. Ifadah Circumambulation
4. Sa`i between Safa and Marwah
There is agreement among Juristic Schools on the obligatory and necessary rituals of Hajj.