Calories in/calories out is such a simple concept - it is the underlying principle behind all weight loss and most of us get it wrong. Unlike other nutritional aspects of food, such as sodium, fat, protein, vitamins, fiber and antioxidants, calories are neither healthy or unhealthy; they are simply a measure of energy. There are different, sometimes conflicting, opinions as to what foods these calories should come from, but the underlying principle remains unchanged.
One of the biggest problems is that we tend to overestimate the amount of calories burned by exercise - especially cardio. My daily 30 min run leaves me dripping in sweat and questioning the meaning of life - and it only burns about the same energy I'll later consume in my breakfast: a large apple, a tablespoon of almond butter and boiled egg. That's why the mindset of "I'll run it off in the morning" or "Well, I'm going to the gym this evening" just doesn't work when reaching for that second helping of pasta or slice of cheesecake.
Calories in/calories out. Put simply, all foods contain calories (energy) our bodies use - even when we are sitting around doing nothing. If we consume more energy than we use, our bodies will store the unused energy in the form of fat. If we balance the amount of energy we take in with the amount of energy we use, our weight will remain the same. And, if we are using more energy than we are consuming, our bodies will pull the energy it needs from our stored supply (fat) and we'll lose weight. That's why you can lose weight on a diet of pizza and fries - or gain weight on a vegan diet.
The attached chart, based on a 150lb woman, will give you an idea of what type, and how much, cardio is required to burn off roughly 200 calories portions of common foods. If you weigh more, you'll burn more calories, and if you weigh less, you'll burn fewer calories doing the same activities, but this should give you a good frame of reference for putting "calories in/calories out" in perspective. If you are interested in learning more about calories and energy balance, you might want to check out this fascinating article.