Alphabetically means in the order of the alphabet: A, then B, then C, and so on. You put names, words, or files alphabetically so people can find them. A short gloss like “relating to the alphabet” is the general idea, but in real use this adverb almost always means “in ABC order”, not merely “about letters”. Teachers, librarians, and office workers use it all the time. It is a clear, slightly formal classroom word. Though actually, when names start with the same letter, you keep going letter by letter — Chen before Cheng — and small words like the are often ignored in titles.