Yeru writes one of the reduced vowels of Old Church Slavonic, the back member of the pair conventionally called the jers, and it carries no numeral value.
The jers are the most consequential letters in the Slavonic alphabet for anyone tracing the history of the Slavic languages. They wrote extremely short vowels which behaved differently according to their position in a word: in some positions they disappeared entirely, and in others they lowered and lengthened into ordinary full vowels. That change, which the literature calls the fall of the jers, reshaped the sound system of every Slavic language, produced the consonant clusters those languages are known for, and happened at different times in different regions.
Because of that, the way a manuscript writes the jers is one of the most sensitive indicators of its date and origin that a philologist has. A text that writes them consistently in the older positions is following an older norm. A text that omits them where they had gone silent, or confuses the two, is recording a language in which the change had already run.
The name is opaque, in this script and in Cyrillic, and no etymology has been established for it.
The letter carries no numeral, standing well past the end of the Glagolitic counted series, and the same is true of its Cyrillic counterpart, since Greek had nothing resembling these vowels and could supply neither a sign nor a number for them. They are among the most purely Slavic parts of the alphabet, invented for a language whose sound system had no Mediterranean parallel.