Rish is the twentieth letter of the Samaritan alphabet and carries the numeral value two hundred. It writes a plain r. The name is the inherited Northwest Semitic one behind Hebrew resh, glossed as a head, which is a reconstruction argued from the word rather than a meaning recorded by anyone writing the script.
The r of the Semitic languages behaves unlike the other consonants in one respect that matters to a reader. In several of the reading traditions it resists doubling, so where the grammar would ordinarily double a consonant and shorten the vowel before it, a word containing this letter lengthens the vowel instead. The effect is systematic, and it is not visible in the consonantal text at all, which makes it one of the many things a reader has to bring to the page rather than take from it.
Greek took the sign as rho and Latin took it from Greek as R, both keeping the sound and the position. The Latin letter acquired its tail somewhere in that transmission, which is why R and rho look related but not identical. The Samaritan form belongs to the other branch and resembles neither.
The character is encoded at U+0813.
The root behind the letter name carries more than the gloss suggests. It means a head, and from that a beginning, a chief, a first thing, and the first word of the Torah, the word the community copies at the head of every scroll it makes, is built on it. The letter is not that word and the name is not a claim about the text; the two share a root, as a great many Semitic words do. It is a fair illustration of how these names work. They are ordinary words of the language rather than labels invented for the alphabet.