I had no real idea what a calorie was, except that miserable people seemed to count them a lot.
I remembered from school that it was an antique unit that measured the energy it took to heat up water. It was clearly time to start investigating these things.
The conventional wisdom is that you take a certain amount of energy in as food, and expend a certain amount in exercise (and just staying alive is quite energy-intensive exercise), and if there's any energy left over then you store it as fat, whereas if there's a deficit, you can use your stored fat to make up the difference.
Eat lots, exercise little -> gain weight
Eat little, exercise lots -> lose weight
It's slightly more complicated than that, in that the energy content of food is variable. You can digest three things, fat, carbohydrate, and protein. A gram of fat is worth 9 calories, a gram of carbohydrate or protein is worth 4.
So 'eat lots' and 'eat little' really mean 'eat lots of calories', 'eat few calories'. Foods with fat in them are to be avoided if you want to lose weight. Foods with carbohydrates and protein should be favoured, because for a given amount eaten, there's more than twice as much energy in the fat.
And the more energy you spend, the faster the weight will come off, so the conventional advice to people wishing to lose weight is 'eat less, exercise more', or in its more sophisticated version, 'eat less, and especially less food with fat in it, and exercise more'.
Unfortunately, whilst the conventional wisdom is a well researched, lovely theory, confirmed in many experiments, and absolutely and uncontroversially true, it's completely useless to people trying to lose weight.