Siddham Letter Bha

It writes the Sanskrit consonant syllable transliterated as bha.

The Latin-letter spelling bha names the syllable; by itself, 𑖥 is not a complete English word. Siddham is an abugida, a script whose consonant letters carry an inherent vowel (here, a). Attached vowel signs change it, while a virama (a mark that removes the vowel) suppresses it.

The recognizable Siddhamātrikā tradition, an early form of Siddham, appears in North Indian records spanning the sixth to tenth century and grew from the Gupta-period Brahmi script, an earlier Indian writing system. By about the eighth century, related regional scripts included Śāradā, Gauḍī/Proto-Bengali, and Nāgarī. Siddham served to write Sanskrit, including Buddhist texts.

What does 𑖥 mean in a mantra?

Usually it gives a Sanskrit sound; the rite may add symbolism.

Indian Buddhists used Siddham to write Sanskrit. With Buddhist learning, the script reached China and Japan, where it remained tied to sutras (Buddhist teachings), mantras, and dharanis (protective recitations).

An example from Japan's Edo period shows bonji—Siddham characters used in Buddhist calligraphy—read phonetically or as a sign associated with a Buddhist deity. The rite and tradition determine which reading applies; a bare 𑖥 does not name a deity.