Spelling was the way that words were formed using a combination of letters or symbols.
Some words were spelled with letters that were silent, such as "knife", which had a silent "k". When preparing to greet the Jarada in 2364, Deanna Troi countered Jean-Luc Picard's criticism of the confusing Jaradan language by noting that Humans spelled knife with a "k". Picard explained that spelled knife with an "n", before adding that he never could spell. (TNG: "The Big Goodbye")
Later, when Jake told Doctor Julian Bashir that he was writing a report of the upcoming battle for the Federation News Service, the doctor reminded him that "Bashir" was spelled with an "i". (DS9: "Call to Arms")