There are different Usages of the letter R.
1. the eighteenth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after Q in a set of items, categories, sizes, etc. e.g. It also denotes the eighteenth of eighteen or more hypothetical people or things.
r = radius as in d=2r where d is diameter.
Besides, R letter of the English Alphabet is used in Romania as the International Vehicle Registration.
In Chemistry R = Gas Constant which is equal to 8.314 Joule Kelvin-1 mole-1 and in Physics R = Electrical Resistance.
The most Common Noun which is used to learn this letter at the Nursery level is Rat; R for Rat.