"Processed food" is food that's been prepared in some complicated way before being sold to customers. Examples include:
- canned soup
- snack bars
- frozen dinners
People think of processed food as being unhealthy. If you want to talk about this kind of food in a more positive way, you can call it "prepared food".
The word "food" is usually uncountable. However, if you're talking about a variety of different kinds of food that you don't eat all at one time, you can call them "foods". So you say:
I ate so much food!
But you can say:
A healthy diet consists of a wide variety of different foods.