Here's a real short video that explains, in simple terms, what a calorie is and why calories in don't equal calories out:
https://www.endocrineweb.com/what-is-a-calorie?utm_source=EndocrineWeb+eNewsletter&utm_campaign=c065385b36-EMAIL_CAMPAIGN_2020_07_30&utm_medium=email&utm_term=0_e6f563893f-c065385b36-49682885&ct=t(EMAIL_CAMPAIGN_2020_07_30)