Industrial processing. As a simple rule, the longer the list of ingredients on the food label, the more processed. The ingredients aren't something you'd normally think of as food? More processed.
Another way I've heard it stated: "single ingredient foods." And as you prepare them, try to keep them that way.
For example, adding a ton of butter to potatoes makes them 10x less healthy. More or less, calorically they become fries. Fries taste great but they should be an occasional treat, not a staple.
Keep in mind that if you buy a bunch of "let's pretend it's not processed" "healthy" foods and use them to cook a meal, you're a) creating an ultra-processed meal, and b) it now has a long list of ingredients, you're just not writing it down.
Eat fresh or frozen meats, vegetables, fruits, some whole grains.
In most supermarkets, shop around the perimeter and avoid everything in the center aisles.