The retroflex sibilant is best known outside Sanskrit for the cluster it forms with the velar stop, which spells a set of words that travelled far: the name of the warrior and ruling class in the classical description of Indian society, the term for release or liberation that names the goal of several Indian religious systems, and the words for a moment, for a field and for the sky. That single cluster carries more weight in the philosophical and social vocabulary of the subcontinent than most whole letters do.
In its own right the letter is largely the output of a rule. In classical Sanskrit the dental sibilant is converted to this retroflex one when certain vowels or consonants precede it, and the grammatical tradition states the conditions exactly, so a reader who knows the rule can predict most of its occurrences. Where it appears without the rule applying, the word is telling the reader something about the language or the scribe.
The letter also stands in the word for a seer, the term for the composers of the Vedic hymns, where it follows the syllabic r that conditions it. Sanskrit words containing the retroflex sibilant frequently also contain an r or a retroflex earlier in the word for exactly this reason, and the pattern is regular enough that a damaged syllable can sometimes be reconstructed from the sibilant that follows it.