What is a calorie?

Last week I saw 'That Sugar Film', and it answered a lot of questions I had about nutrition, sugar especially. Then this short TED ED popped up in my feed, explaining what calories are and how they work. I thought it was pretty insightful and maybe they should've included a bit about this in That Sugar Film? Oh well, thought this was a nice piece of the nutrition puzzle I was till missing. Solved now, thanks.