What is the letter "C" for the element carbon?
Not much to say other than it's the chemical symbol for carbon. It's from the latin, carbo, meaning coal. It's just a convenient way of denoting carbon atoms in a chemical compound or molecule.
#"CH"_4#conveys the structure of methane, without having to explain what "methane" means. #"CH"_3"CH"_2"OH"#conveys the structure of this two-carbon alcohol, instead of requiring one to explain what "ethanol" means.