The very first construction of the Kaaba was done by Prophet Ibrahim
(peace be upon him) and his son Ishmeel as a house of worhip to one God.
Allah (the Glorified and Exalted) says in the Quran that this was the
first house that was built for humanity to worship Allah (the Glorified
and Exalted). Soon after Prophet Ibrahim (peace be upon him) died, people
started filling the Kaaba with idols. It was not until Prophet Muhammad
(peace and blessings be upon him) defeated the idol-worshippers that the
idols inside the Kaaba were destroyed.
One of the Five Pillars of Islam requires every Muslim to perform the Hajj pilgrimage at least once in his or her lifetime if able to do so. Multiple parts of the Hajj require pilgrims to walk seven times around the Kaaba in a counter-clockwise direction. This circumambulation (Tawaf), is also performed by pilgrims during Umrah.
Muslims around the world face the Kaaba during prayers, no matter where they are. This is called facing the Qiblah.
As mentioned in this verse in the Quran, "Verily, We have seen the turning of your (Muhammad's) face towards the heaven. Surely, We shall turn you to a Qiblah (prayer direction) that shall please you, so turn your face in the direction of Masjid al-Haram. And whosesoever you people are, turn your faces (in prayer) in that direction." [2:144 ].