Al-Aqsa Mosque is the second Mosque placed on earth.

 Abu Dharr said:

'I said, 'O Messenger of Allah, what is the first Mosque placed on the earth?' He said: 'Masjid al-Haram'. I then asked: 'What is the second Mosque?' He said: 'Al-Aqsa Mosque'. I asked: 'What is the time period between the placements of these two Mosques?' He said: 'Forty years, and whenever you have to pray perform prayer for the entire earth has been made a place of prayer.' (Muslim)

It was built by Prophet Solomon, son of David. Abdullah b. Amr b. al- Aas reported that Amr b. al-Aas said that the Messenger of Allah said:

'After Solomon, son of David, finished building Mosque al- Aqsa, he asked Allah to bestow him a Rule and Kingdom like no one had before, it not befitting anyone afterwards, and that no one would come to this Mosque seeking to pray in it, except that he would return from it sin-free.' The Prophet said: 'As for two (of the three requests) he was granted, and I hope that I will be granted the third.' (Ibn Khuzaimah)

It is the first of the two Qiblah (direction of prayer) the Prophet faced in his prayers and the Muslims prayed towards it before they were ordered to pray facing the Kabah. During the Night Journey, he stopped at Mosque al-Aqsa, and from there he ascended to the Heavens. He led the prophets in prayer during this night journey.