Miqat refers to the boundary where it becomes necessary for pilgrims to adorn the Ihram garments and impermissible to pass except in the state of Ihram. Those who cross the boundary without having entered into the state of Ihram must perform an animal sacrifice (Damm) as expiation.

Narrated Ibn `Abbas:

Allah’s Messenger (saw) had fixed Dhul Hulaifa as the Miqat for the people of Medina; Al-Juhfa for the people of Sham; and Qarn Ul-Manazil for the people of Najd; and Yalamlam for the people of Yemen. So, these (above mentioned) are the Mawaqit for all those living at those places, and besides them for those who come through those places with the intention of performing Hajj and `Umra and whoever lives within these places should assume Ihram from his dwelling place, and similarly the people of Mecca can assume lhram from Mecca. Sahih al-Bukhari 1526

An additional Miqat was affixed during the reign of the second caliph, Umar Ibn al-Khattab رضي الله عنه after the towns of Basra and Kufa had been captured:

Narrated Ibn `Umar: When these two towns (Basra and Kufa) were captured, the people went to `Umar and said, “O the Chief of the faithful believers! The Prophet (saw) fixed Qarn as the Miqat for the people of Najd, it is beyond our way and it is difficult for us to pass through it.” He said, “Take as your Miqat a place situated opposite to Qarn on your usual way. So, he fixed Dhatu-Irq (as their Miqat).” Sahih al-Bukhari 1531.

Sources: https://hajjumrahplanner.com/ and https://sunnah.com/