Kissing the Black Stone is an act of worship in which a Muslim kisses a stone to which he has no connection except worshipping Allah by venerating it and following the Messenger of Allah (peace and blessings be upon him) by doing that.

Tawaf al-ifadah and sa`i are two of the pillars or essential parts of Hajj, without which Hajj is not complete. If someone failed to do them, then his Hajj is not complete, and he must do them.