Yu writes the sequence of a glide followed by the vowel u, and it carries no numeral value, standing past the end of the Glagolitic counted series.
Old Church Slavonic distinguishes a plain vowel from the same vowel preceded by a glide, and it distinguishes them consistently, so the alphabet needed signs for the combinations as well as for the vowels. Several such signs occupy the tail of the order, this one among them, and their presence is one of the clearest indications that the alphabet was designed for the language rather than adapted to it. Greek had no comparable need and no comparable signs, so nothing here could be inherited.
The letter is common, because the sequence it writes is common in Slavonic endings and in the pronouns and particles the language uses constantly. That makes it one of the well attested forms in both the rounded and the angular hands.
Its name is opaque. Like every letter name in this part of the alphabet it is recorded in the reference literature without a gloss, and the atlas carries an open question about the whole set rather than proposing a reading for any single one of them.
The letter also has a long afterlife. It passed into the Cyrillic tradition and from there into the modern orthographies of several Slavic languages, where the descendant sign still writes the same glide and vowel together. Very few of the letters at this end of the Glagolitic order survived that well, and the ones that did are the ones whose sounds the languages kept.