Tajweed, the set of rules directing the way in which the words of the Quran should be pronounced during its recitation.
The word “Tajweed” means to develop, to do better. It means reading a Quran in a proper way it means Tajweed.
Tajweed of the Holy Quran is the knowledge and application of the laws of recitation, so the reading of the Holy Quran is as recited by our beloved Prophet Muhammad (Peace and Mercy of Allah be upon him).
The Qur'an was revealed with Tajweed rules applied to it. In other words, when the angel Jibreel (Gabriel), may Allah exalt his mention, recited the words of Allah to the Prophet Muhammad, Sallallahu 'alaihi wa Sallam, he recited them in a positive way and he showed the Prophet, Sallallahu' alaihi wa Sallam, the ways in which it was permitted to recite the Qur'an. So it is obligatory upon us to follow those rules so that we recite it in the way it was revealed.
At the time of the Prophet, sallallaahu 'alaihi wa Sallam, there was no need for people to read Tajweed because they talked with what is now known as Tajweed, so it was normal for them. When the Arabs began mixing with the non-Arabs and as Islam spread, mistakes in the Qur'anic recitation began to appear, so the scholars had to record the laws. Now, because the common Arabic that Arabs speak has changed so much from the Classical Arabic with which the Qur'an was revealed, even the Arabs have to study Tajweed.