This pretty much means eating non-processed food. So no soda, no "pseudo-juice" like SunnyD, no candy bars besides straight-up squares of dark chocolate, no pre-packaged meals, no "snack foods", no "diet-food"...pretty much avoiding anything that comes in a box.
You're gonna be shopping on the outside of the grocery store for most of your stuff/
So pretty much if you're eating clean you're gonna be eating mainly fresh produce (canned is OK on occasion depending on what canned foods you're gonna be eating), fresh cuts of meat (aka, not things like hot dogs or sausages), dried grains that you gotta cook yourself (like rice and pasta)...and the most "processed" thing you're gonna be eating is dairy products.
Ow, that is a highly debatable topic whose definition varies from person to person. The gist is you avoid processed foods, you eat fresh meats and produce, lean meats or fat-trimmed beef, and whole grains. Avoid all junk foods. In addition, when you cook, you use only the healthy oils (think extra virgin olive oil) in light quantities, or you bake the foods.
Should mention, her late husband published a magazine, which contained the deep-level explanations of human physiology and metabolism, which I applied and lost around 90lbs of fat, put on around 15lbs of muscle (estimated by body fat percentage) in little over four months, and I used her plan to eat.