The less processed the better
Humans process foods when we “do” things to them. Some processing is fairly simple and uses ancient techniques (eg – baking a potato), while other processing is much more complex and uses modern, industrial techniques (eg – making a Mars bar).
As a very rough rule of thumb, unprocessed foods could have come almost directly from the farm, field, forest or sea. Processed foods would have to go via a factory before reaching you.
Here are some examples of foods with different levels of processing: