Small yus writes a nasal vowel, the front member of the pair the Slavonic tradition calls the yuses, and it carries no numeral value.
Old Church Slavonic had two nasal vowels, a front one and a back one, inherited from an earlier stage of Slavic in which certain vowels followed by a nasal consonant had fused into a single nasalised sound. The alphabet gives each a letter, and the letters are among the most characteristic features of the script.
The sounds did not survive. Every Slavic language except Polish lost the nasal vowels, denasalising them into ordinary vowels at various dates, so an early Slavonic manuscript records a distinction that almost no living Slavic speaker makes. That is precisely what makes the letters valuable: a word written with a yus can be traced to an original nasal vowel even where every descendant language has lost it, and the correspondences between the yus letters and the vowels of the modern languages are a standard tool of comparative Slavic work.
The name is opaque and the reference literature records it without an etymology, in this script and in Cyrillic alike.
The letter carries no numeral in either script. Greek had no nasal vowels and so had no letter and no number to lend, and the Glagolitic count had already ended some way earlier in the order. The nasal signs are among the most purely Slavic parts of the alphabet, invented for sounds that no script in the Mediterranean world had ever needed to write.