Extra food means exactly that. Not part of your lunch, not trail snack food, but food that’s extra in the sense that you’ll bring it back to the car or campsite at the end of the trip. It’s food that’s not part of your hike’s food menu. It’s food that’s there to help sustain you in the event of an emergency.
Extra food remains in your pack, uneaten, ready to go for the next trip.
Extra food should be compact and lightweight, yet high in calories, vitamins, and protein. Temporarily forget about the diet or weight-loss plan. If you need food during an emergency, such as an inadvertent overnight stay, you need short-term body fuel to get you by until help arrives.
A good choice is a quantity of athletic high-energy bars (such as “Power Bars”, etc.), or similar nutritional products. Another choice (although a little bulkier and heavier) is a sealed high-energy, high-calorie military MRE (Meal Ready to Eat).
Whatever you choose, your extra food should sit at the bottom of your pack, out of the way, yet ready to go if you should need it.