The area between Hajar al-Aswad and the Kaaba Door is called Multazam. It is about two meters wide, and this is the place where du’as (prayers) are accepted.
Multazam is an area where duas are accepted. It is a sunnah of Prophet Muhammad to hold onto the wall of the Kaaba so that the cheek, chest and arms are against the wall. It is reported that Abdullah bin Umar (may Allah be pleased with him) once performed a tawaf, performed a salah, and then kissed the Hajar al-Aswad. After that, he stood between Hajar al-Aswad and the door of the Ka'ba in such a way that his cheek, chest and arms were against the wall. Then he said: “This is what I saw Rasulullah (saw) do.”
Abdullah bin Abbas (may Allah be pleased with him) says: "Signs of acceptance of any dua between Hajar al-Aswad and the door of the Kaaba will surely be visible."
Mujahid (may Allah be pleased with him) says: “The area between Hajar al-Aswad and the door of Kaaba is called Multazam. Allah will give man everything he asks for and will save him from everything he seeks from there. ”
Source: The History of Makkah Mukarramah – Dr. Muhammad Ilyas Abdul Ghani