What is meant by Sidratul-Muntaha that the Quran has mentioned?
According to what experts of the Arabic language say, the word “Sidrah” means a tree full of leaves with a great shade, and the term “Sidratul-Muntaha” refers to the great tree full of leaves with a great shade located at the highest point of the ascension of the angels and the souls of the shuhada and the knowledge of the prophets and the actions of the people, a place where the Lord’s angels cannot pass, and even Jibra’il stopped at during the mi’raj of the prophet (pbuh).
Although the Quran hasn’t explained about Sidratul-Muntaha, it has been spoken of and described in Islamic traditions, all of them uncovering the truth that this term is merely a simile used because of the lack of words for describing such great realities.
In a hadith by the prophet (pbuh), it has been narrated that: “رایت علی کل ورقة من اوراقها ملکا قائما یسبح الله تعالی” meaning: “I saw an angel standing on each leaf praising Allah (swt).”  In another hadith it has been narrated that the prophet (pbuh) said: “انتهیت الی سدرة المنتهی، و اذا الورقة منها تظل امة من الامم”, meaning: “I reached the Sidratul-Muntaha and saw that under the shade of each leaf, there was a nation.” 
These phrases signify that the Sidratul-Muntaha isn't just any ordinary tree like the ones we see in this world, but refers to a great shade close to Allah (swt) in which has angels praise Him on the leaves of and nations of the good and righteous reside under. 
In the tafsir of Al-Mizan, it has been said about the Sidratul-Muntaha that: “As for what this tree is, we didn’t find anything in the Quran that says anything about it, and it seems as if Allah (swt) didn’t want to explain this part and intended to leave it unclear. The phrase “إذ یَغشی السدرة ما یغشی” (when what covers the Sidrah, covers it) confirms this claim, because in it something hidden and unclear is spoken of. In hadiths, Sidratul-Muntaha has been interpreted as a tree above the seventh heaven in which the actions of Allah’s (swt) servants go up till. 
 Tabarsi, Majma’ul-Bayan, under verse 14 of surah Najm, “عند سدرة المنتهی
 Nurul-Thaqalain, vol. 5, pg. 155.
 Tafsir Nemouneh, vol. 22, pg. 498.
 Seyyed Muhammad Husein Tabatabai, Al-Mizan (Farsi translation by Seyyed Mohammad Baqir Hamedani), vol. 19, pg. 49.