This letter with its inherent vowel, lengthened, is the ordinary Sanskrit word for or. Like the word for and it is an enclitic, placed after the item it governs rather than between the alternatives, so a Sanskrit choice runs item, item, or rather than item or item. The two particles behave alike and are learned together, and both are among the commonest words in the language and among the first a reader of inscriptions comes to recognise.
The letter is the semivowel that the vowel u turns into whenever a dissimilar vowel follows, which is the back-series counterpart of the rule that turns i into ya, so it appears constantly at the seams between words as well as inside them. It also opens a substantial family of ordinary vocabulary, including the roots meaning to speak, to blow and to dwell.
Its actual pronunciation runs between a lip-and-teeth sound and a two-lipped one depending on region and period, and neither Brahmi nor the classical phonetic descriptions draw the distinction, so the single letter covers both. That is one of several places where the script is less specific than a modern reader would like, and it is the ordinary economy of a writing system used by people who already knew the language.