Shta writes a cluster, the sequence conventionally transliterated sht, and it carries no numeral value. It belongs to the Slavic end of the alphabet, where the letters record sounds Greek did not have and the counted series has already stopped.
The sound is one of the clearest dialect markers in the whole of Slavic. The cluster this letter writes developed from an earlier consonant combination, and the different Slavic languages resolved that combination in different ways: the South Slavic reflex that Old Church Slavonic records is not the one that West or East Slavic produced. A word written with this letter in an early Slavonic manuscript therefore appears with quite different consonants in the modern languages, and the correspondences are among the first things a student of comparative Slavic learns.
That makes the letter useful evidence about the language of the surviving canon. The manuscripts that preserve early Glagolitic writing come from the south, from Bulgaria and Macedonia and later from the Adriatic coast, and the reflex this letter records is the southern one. Whether the alphabet as first designed for Moravia handled the sound the same way is a separate question, and the surviving evidence cannot answer it directly.
Its name is opaque, like the other names at this end of the order.