https://en.wikipedia.org/wiki/Nova_classification
The tl;dr of ultraprocessed is that it's those exclusively industrial foods prepared with ingredients you wouldn't stock in your pantry (high fructose corn syrup, artificial flavors, protein isolate, etc)
In part, these foods are bad because they are high in sygar and salt. But that's not the point of the classification, which intentionally takes a step back from the old approach based of mesuring the individul ingredients and macronutrients. The central thesis is that, for public health policy it's easier to frame it as a discussion of traditional food vs industrial junk, and how ultraprocessed foods are replacing traditional foods. The classification doesn't want to get lost in the details of ingredients, which the industry has always been adept at gaming anyway (fat is the boogeyman -> replace it with sugar, add vitamins to make it sound healthier, etc)