The thermic effect of food (TEF) is the energy expenditure above the body's resting metabolic rate due to the cost of processing food for use and storage. Basically, your body has to burn calories to break food down. Your system has to work much harder to assimilate nutrients that aren't processed.
The TEF of food accounts for 10% of your total calorie intake. If you consume 2,000 calories per day, your body will burn around 200 of those calories digesting food.
This is why it's so important to eat a lot of protein when dieting. Not only will you maximize muscle retention, but you'll also burn more calories due to a higher thermic effect of your food.
The quality of the food you consume greatly matters. Foods with a higher thermic effect also tend to be pretty satiating. Staying in a caloric deficit will be a heck of a lot easier if what you're eating makes you feel full and satisfied.
If you're eating lean sources of protein, complex carbs, and healthy fats, a caloric deficit won't be as hard to achieve as it would be if you were chowing down on Cosmic Brownies, Twinkies, and doughnuts. Those have a low TEF, a high-calorie content, and they're not satiating.
Not that I'm trying to crap on doughnuts. Who doesn't appreciate a good doughnut?