Sikshadeals with the study of pronunciation of words and syllables through the correct intonation,  conjunction (sandhi) and disjunction (vichheda) of syllables, recognized  primarily as vowels and consonants. It intends to train the students in the art and science of articulation of words and syllables so that they can chant the vedic hymns perfectly, producing the desired sound vibrations and maintain the ritual purity andefficacy of the ceremonies they perform. 

A lot of importance was attached in ancient India to correct pronunciation of the Vedic hymns because of the belief that the Vedas  were inviolable and divine in origin. The scriptures proclaimed that a Sanskrit syllable was in the form of a syllable (askhara Brahman), an imperishable entity (a + kshara), revealed to the mortals for the sake of dharma and welfare of the world, just as the thunder that rumbled from the heavens or the subtle sounds that could be heard in meditation. Its medium was ether (space), the medium of  gods So no liberties could be taken or  transgressions be made by mere mortals while chanting them, without attracting unhappy consequences for  themselves and those for whom the were sung. As a result of these beliefs, Sikhsa developed into a separate branch of study to preserve the integrity and purity of the divine words and save the dharma from human fallibility. 

The teachings of the Siksha are contained in the ancient texts known  as Pratisakhyas, each attached to a particular Samhita, providing instructions for the recitation of the hymns contained in it. The Pratisakhyas were probably composed by many grammarians like Saunaka before Panini and revised from time to time. Siksha played an important role in vedic India at a time when there was no written script and the knowledge of the Vedas had to be transmitted from one person to another orally. By  establishing the ground rules of proper pronunciation, it minimized the   chances of distortion that would usually accompany verbal communication. Vedic students were required to spend years reciting the mantras using the principles enunciated in the Pratisakhyas under the careful watch of their teachers before they were allowed to return to their homes and become ordained priests. As young apprentices they could ill afford to risk their careers by deviating from established traditions or show any sign of incompetence or imperfection due to lack of mastery.