Something non-essential is not something you completely need. You can get by without it, at least for a while. Food, water, medicine, and keeping the lights on are essential; a new phone case, a holiday, or a haircut can be called non-essential. The word got loud during lockdowns, when governments said which shops and jobs were “essential” and which were not — though actually that was a policy label, not a moral one: a bookshop can matter a lot to you and still be non-essential in that official sense. In ordinary talk it just means “nice to have, not must-have,” and it is a bit more formal than “not that important.”