A remarkable share of the central vocabulary of Indian religion begins with this letter. The name of one of the two great deities of classical Hinduism, the word for the divine power personified as a goddess, the term for what is heard and therefore for revealed scripture, the word for peace that closes so many recited formulas, and the honorific prefixed to names, titles and texts across the subcontinent all open with it. So does the term for a corpse and the term for a hundred, which is a reminder that the letter is an ordinary sound and not a sacred one.
In the grammar it participates in a regular substitution. The dental sibilant becomes this palatal one before a palatal consonant, which is one of the routine rules of junction, so a word ending in the dental sibilant followed by a word beginning with a palatal shows this letter at the seam. Reading a continuous text in the script therefore means undoing the substitution to find the underlying forms.
The transliteration used in European scholarship writes it with an s carrying a diacritic, and the popular romanisation writes sh, which collides with the retroflex sibilant that is also written sh. Two distinct Sanskrit sounds therefore reach a general reader as one spelling, and the confusion runs deep enough that many familiar words are habitually pronounced with the wrong sibilant outside South Asia.