the short answer
An online nutrition coach is a professional who helps clients improve how they eat - through remote meal planning, macro or calorie targets, habit coaching, and regular check-ins delivered over an app rather than in person. They coach general healthy eating for non-clinical clients, and refer clinical nutrition questions to a registered dietitian or physician.
This article is general information for coaches, not legal, medical, or nutrition advice - certification rules, protected titles, and scope of practice vary by country and change over time, so verify the specifics for your situation.