What is Hajj, the pilgrimage to Mecca?

فِيهِ ءَايَٰتٌۢ بَيِّنَٰتٌۭ مَّقَامُ إِبْرَٰهِيمَ ۖ وَمَن دَخَلَهُۥ كَانَ ءَامِنًۭا ۗ وَلِلَّهِ عَلَى ٱلنَّاسِ حِجُّ ٱلْبَيْتِ مَنِ ٱسْتَطَاعَ إِلَيْهِ سَبِيلًۭا ۚ وَمَن كَفَرَ فَإِنَّ ٱللَّهَ غَنِىٌّ عَنِ ٱلْعَٰلَمِينَ ﴿٩٧﴾

Wherein are plain memorials (of Allah's guidance); the place where Abraham stood up to pray; and whosoever entereth it is safe. And pilgrimage to the House is a duty unto Allah for mankind, for him who can find a way thither. As for him who disbelieveth, (let him know that) lo! Allah is Independent of (all) creatures.

وَأَذِّن فِى ٱلنَّاسِ بِٱلْحَجِّ يَأْتُوكَ رِجَالًۭا وَعَلَىٰ كُلِّ ضَامِرٍۢ يَأْتِينَ مِن كُلِّ فَجٍّ عَمِيقٍۢ ﴿٢٧﴾

And proclaim unto mankind the pilgrimage. They will come unto thee on foot and on every lean camel; they will come from every deep ravine,

۞ إِنَّ ٱلصَّفَا وَٱلْمَرْوَةَ مِن شَعَآئِرِ ٱللَّهِ ۖ فَمَنْ حَجَّ ٱلْبَيْتَ أَوِ ٱعْتَمَرَ فَلَا جُنَاحَ عَلَيْهِ أَن يَطَّوَّفَ بِهِمَا ۚ وَمَن تَطَوَّعَ خَيْرًۭا فَإِنَّ ٱللَّهَ شَاكِرٌ عَلِيمٌ ﴿١٥٨﴾

Lo! (the mountains) As-Safa and Al-Marwah are among the indications of Allah. It is therefore no sin for him who is on pilgrimage to the House (of Allah) or visiteth it, to go around them (as the pagan custom is). And he who doeth good of his own accord, (for him) lo! Allah is Responsive, Aware.