Big yus writes the back nasal vowel, the second member of the pair of nasal vowels that Old Church Slavonic inherited, and it carries no numeral value.
The vowel it records was lost across almost all of the Slavic world. Where it stood, the modern languages have an ordinary oral vowel, arrived at by denasalisation at various dates in various places, and only Polish retains nasal vowels of its own. A word written with this letter in an early manuscript can therefore be traced to a nasal vowel in the parent language on the strength of the spelling alone, which is the sort of leverage that makes an early orthography worth studying in its own right.
The letter is common in the canon, because the vowel it wrote occurred in frequent endings, so its forms are well attested in both hands of the script.
Its name is opaque and the reference literature records it without an etymology. The atlas treats that, as it treats the rest of the names at this end of the alphabet, as a gap in the record rather than as an invitation to supply a reading, and it carries an open question stating what a study of the whole set would have to establish.
No number attaches to it in Glagolitic. The counted series in this script stops at the second letter for the vowel o, and everything after that, including both nasal vowels and all four nasal signs, stands outside the arithmetic. Cyrillic leaves the same letter unnumbered for its own reason, that Greek had nothing in the slot to inherit from.