The miqat refers to the boundary points the Prophet ﷺ designated around Mecca, beyond which anyone intending Hajj or Umrah must not pass without first entering the state of ihram. In the well-known hadith of Ibn 'Abbas he designated four: Dhul-Hulaifah for those coming from Madinah, Al-Juhfah for Syria, Egypt and the west, Qarn al-Manazil for Najd, and Yalamlam for Yemen, adding that anyone else crossing these points, even if not from those regions, should enter ihram there too (Sahih al-Bukhari 1526). A fifth, Dhat 'Irq, serves the people of Iraq: it is reported that the Prophet ﷺ himself designated it (Sunan Abu Dawud 1739 — graded Sahih by al-Albani), while in Sahih al-Bukhari 1531 it was 'Umar ibn al-Khattab who fixed it once Kufa and Basra were settled — either way it is an established miqat. Today, air travelers are advised to make the intention and wear ihram before their flight crosses over the relevant miqat.
If someone passes their miqat without ihram — forgetfully or through unfamiliarity — the majority of scholars hold they should return to the miqat and enter ihram from there if possible. If they don't return and instead enter ihram later, closer to Mecca, their Hajj or Umrah remains valid, but they owe a dam (sacrificing a sheep or goat, distributed to the poor of the Haram) as expiation for the omission. Acting deliberately makes it more important to correct the matter promptly rather than relying on the expiation alone.