Aerial view of the Kaaba and Masjid al-Haram
The Kaaba is the first house of Allah (the Glorified and Exalted) built by Prophet Ibrahim (peace and blessings be upon him). Muslim, Christians, and Jews are related by being the descendents of Prophet Adam (peace and blessings be upon him). Muslims touch the Kaaba to feel closer to Allah (the Glorified and Exalted). Collective circumambulation of the Kaaba fosters unity between Muslims.
Timelapse of the Kaaba, Masjid al-Haram, and other sites in Mecca