Ultra-processed foods are foods that really don't resemble their original state anymore. For example, if you take a piece of chicken, add additives, process it, add emulsifiers, etc., you end up with chicken nuggets. Chicken nuggets are ultra-processed foods. They contain really not that much of the original chicken anymore.
That's one example of ultra-processed foods. It also includes sugary drinks, other ready-made or frozen meals, burgers, some sandwiches, etc.