Pali is a northern Indian language which has been in continuous use in one form or another for approximately 2500 years. Originally a mainly spoken vernacular language, it eventually evolved into one of the most important and influential literary languages in the world.
Pali is the language used to record the original discourses of the Buddha (or at least the closest thing we will ever have to them), along with a large collection of supplementary commentaries and other texts which are together known as the Pali Canon. The Pali Canon is the scriptural basis of all forms of Buddhism, and of Theravadan Buddhism in particular.
With the spread of Theravadan Buddhism throughout south and south-east Asia, the Pali language came to have a large impact on the languages of regions including modern Sri Lanka, Myanmar, Thailand, Laos, Cambodia and southern Yunnan.
There are at least three reasons why Pali is a valuable and rewarding language to study. A.K. Warder addressed what is probably the most common reason in the preface to his Introduction to Pali (1963), arguably still the most useful Pali primer available in English today:
The study of early Buddhism will always be the objective of the majority of those who take up Pali, though in addition to the interest of the language itself we must stress the importance of the texts for the study of the history of Indian literature, especially secular literature.
Buddhism has been the subject of the most varied fantasies in the West. The few reliable guides are overlooked in the mass of claptrap, humbug, and pure fiction. Inevitably serious work is less readable than journalistic antithesis and exoticism. Moreover in the field of early Buddhism and Pali studies, at least, even the most serious scholars have remained obstinately various in their interpretations. It must still be said that the Pali texts themselves are the only reliable authority as to their meaning. The further advance of these studies depends on the deeper analysis of these texts. Rather than add another volume to the bewildering mass of books on “Buddhism”, it seems more constructive to open a door directly on the Pali.
Today, an even more bewildering mass of websites, videos, apps and other online resources on “Buddhism” exist in addition to the books, so Warder’s original rationale is now more valid than ever.
A second reason to study Pali is that it was used as a learned literary language and lingua franca across several cultures for many hundreds of years. This means that, in addition to recording the Buddha’s discourses, a treasure-trove of Pali manuscripts also exist including “secular” material as diverse as historical chronicles, travelogues, grammar and linguistics and medical texts.
A third reason to study Pali involves linguistics: on the one hand, Pali is an Indo-European language with obvious similarities to Latin languages among many others, while on the other it has had a significant impact on most south and south-east Asian languages.