Can someone help me with calories?

I have a little trouble understand how Calories work. Today I ate potato salad (290 cal) and 100 cal baked chicken. that's all I ate. I don't want to gain weight so I walked 2 miles to sort of burn it off. Does exercising every day help? If I buy something from the store and it contains 500 calories, but the fat calories are 150, which do I go by?