It was the first mosque built by Allah's Messenger (pbuh) in Al-Madinah when he arrived there as a emigrant and Allah has mentioned it in the Glorious Qur'an:
"Verily, the mosque whose foundation was laid from the first day on piety is more worthy that you stand therein (to pray)." The Holy Qur'an, Chapter 9, Verse 108

When the Prophet(pbuh) migrated to Al-Madinah, he stopped in Quba at the house of Kulthum bin Al-Hadm and he took his camel-pen and built a mosque and he took part in the building of it with them and he prayed therein and it is confirmed that it is the first mosque in which he prayed with his Companions openly in congregation.

The Qibla of the Quba mosque was, at first, towards Jerusalem, until Allah (swt) commanded His Messenger to pray towards the Ka'bah, so they wished to rebuild the mosque and the Prophet(pbuh) came to them and marked the Qiblah and participated with them in the building of it.

The Quba mosque has received much attention from the muslims and their rulers. The mosque now has 56 small domes and 6 large domes and 4 minarets and can hold twenty thousand worshippers.