Sha writes the consonant sh and carries no numeral value, standing at the Slavic end of the order beyond the counted series. It is among the letters that had to be devised rather than borrowed, since Greek has no sign for the sound and no letter that could be pressed into service for it.
The sound arose in Slavic from the softening of an original s and from certain consonant clusters, and it is frequent, appearing in common endings and in a large share of the vocabulary. That makes the letter one of the more visible shapes on a page and one of the ones a reader of the script learns early despite its position near the end of the alphabet.
The name is opaque, and no etymology has been established for it in either Slavonic script.
The shape is the single most discussed Glagolitic form in the argument about where the letters came from, because it resembles the Hebrew letter shin closely enough that the resemblance has been remarked on for a very long time, and because both letters write a sh sound. The atlas records that as a debated relation, with the competing explanations named, and does not treat the resemblance as a demonstration.
The Cyrillic letter for the same sound is generally described in the same terms, so whatever is decided about one sign has consequences for the other. That is a recurring feature of this stretch of the alphabet: the two Slavonic scripts had the same problem to solve, solved it in the same place in the order, and the evidence for how either of them solved it is thin.