You put "whatnot" at the end of a list when you have given enough examples and do not want to name the rest. "We packed towels, sunscreen, snacks and whatnot." It tells the listener: there was more of this same kind of stuff, you can imagine it yourself. It nearly always follows "and", and it nearly always comes last. It is friendly, casual speech — the sound of someone chatting rather than writing carefully. In an essay or a report you would write "and so on", "and other things", or "etc." instead. "Whatnot" also carries a slight shrug with it; it can suggest the remaining items are small, ordinary, or not worth listing. Sometimes that shrug tips into gentle dismissal — "he's always busy with meetings and whatnot" can hint that you do not take those meetings very seriously. Say it as two clear syllables, WHAT-not. (There is also an old, unrelated-feeling use where a whatnot is a small set of shelves for displaying ornaments, but you will only meet that in old novels or antique shops.)