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. He appointed five: Dhul-Hulaifah for those coming from Madinah, Al-Juhfah for Syria, Egypt and the west, Qarn al-Manazil for Najd, Yalamlam for Yemen, and Dhat 'Irq for Iraq, adding that anyone else crossing these points, even if not from those regions, should enter ihram there too (Sahih al-Bukhari 1526). 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.