Ritsi writes the consonant r and takes the numeral one hundred, the first of the hundreds in the Glagolitic count. Its name is an imperative, say, and it opens the last stretch of the recitation whose sense is generally agreed: ritsi slovo tvrido, say the word firmly.
The sound is a trill, and in Old Church Slavonic it can carry a syllable on its own, standing between consonants where a modern English speaker would expect a vowel. The writing system handles that by writing the r with one of the reduced vowel letters beside it, which is not the same as recording a vowel that was pronounced, and the correct reading of such sequences is a standing question in the phonology of the language.
The numeral one hundred is reached by counting the Glagolitic letters in their own order, this being the nineteenth letter. From here the alphabet counts in hundreds to the end of its numbered series.
At this letter the two Slavonic numeral systems briefly agree again, which is a coincidence of where the unnumbered letters fall in the Cyrillic count rather than a return to a common scheme. They agree for a short run and then part for good at the end of the alphabet.
The letter is also one of the most frequent in the script, since Slavonic uses r heavily in its verb endings and in a large share of its vocabulary, so it is among the shapes by which a hand is most readily characterised. In the angular Croatian tradition it takes a form that a reader of the rounded books has to learn again, and the two forms sit at opposite ends of several centuries of continuous use in one liturgical tradition.