Hajj is the pilgrimage to Mecca and the fifth of the Five Pillars. Once in a lifetime, every Muslim who is physically and financially able is required to make it.
"Islam is built upon five [pillars]: … the pilgrimage (Hajj) …"
— Sahih al-Bukhari 81
A duty for those who are able
"And [due] to Allah from the people is a pilgrimage to the House — for whoever is able to find thereto a way."
— Quran 3:97 (Saheeh International)2
A call going back to Abraham
The Quran links the pilgrimage to the Prophet Abraham (Ibrahim), who with his son raised the foundations of the Kaaba — the cube-shaped House of worship at the center of the rites (see Abraham (Ibrahim) in Islam):
"And [mention] when Abraham was raising the foundations of the House and [with him] Ishmael, [saying], 'Our Lord, accept [this] from us.'"
— Quran 2:127 (Saheeh International)5
"And proclaim to the people the Hajj [pilgrimage]; they will come to you on foot and on every lean camel; they will come from every distant pass."
— Quran 22:27 (Saheeh International)3
When and how
Hajj takes place once a year, during the month of Dhul-Hijjah, and brings millions of pilgrims together at the same time and place.678 Pilgrims enter a state of consecration (ihram, marked by simple white garments that erase distinctions of wealth and rank) and perform a series of rites — including tawaf, circling the Kaaba — which the Quran instructs them to complete for God:
"And complete the Hajj and umrah for Allah."
— Quran 2:196 (Saheeh International)4
In short
Hajj is the pilgrimage to Mecca — a once-in-a-lifetime duty for those able, performed each year by millions standing equal before God, in rites that reach back to the Prophet Abraham.