Tsha is the eighteenth of the thirty consonants and the aspirated partner of tsa: a ts released with a breath. Its shape follows the rule of its line, being the palatal letter cha carrying the tsa phru, the small mark that turns a palatal into a dental affricate.
English has no aspirated ts and an English ear tends not to register the difference between tsa and tsha at all. That is a standing difficulty for learners, because the pair is contrastive in Tibetan and because so much of the modern language rests on distinctions of exactly this kind. In modern Lhasa speech both letters carry high tone, so unlike the pairs in the earlier lines they are not separated by pitch either; the aspiration is the whole of the difference.
Tsha is a root letter and takes no orthographic office. Like tsa and dza it belongs to the part of the alphabet Tibetan built for itself, which means that a reader working through the recitation has by this point passed out of the inherited grid and into the language of the adapters.
Tsha takes superscripts and the wa subscript. It is met far more often in native vocabulary than dza, its voiced companion in the same line, which a reader can go a long way without seeing at the head of a word at all, so the three affricates are very unequal in how often they are used despite being learnt together.