When Islam was being spread (and it was done so at a very quick pace and also into non-Arab speaking countries) not everyone's tongue was accustomed to the Arabic letters and sounds. Thus, when reciting the Qur'an, much error and distortion occurred and the Muslim scholars feared (the perpetration of) that error and distortion.
It was at this point that some of them recorded the rules and foundations that regulate the correct pronunciation of Qur'an, and they named this the Science of Tajweed.
The rules were not made up by these Scholars. In fact, all they did was closely observe the perfect readers who read as they were taught by the Prophet (peace and blessings be upon him) and wrote down for later generations the rules of recitation of the earlier generations.
From the outset, Tajweed was a Science that cannot be learnt only from a book and will always retain this inherent quality.
The most important part of Tajweed is learning about correct positions of the organs of speech and the manner of articulation. The Qur'an can lose its meaning if the letters are not pronounced correctly.