It’s best to think of them as 1 calorie equals to 1 unit of energy. This energy is used by our body to perform various tasks like keeping us alive, walking, lifting weights etc. We all need different amount of calories because of our lifestyle, weight, height, genetics and activity levels.
A person that lifts heavy weights 5 times a week will need more than somebody that has an office job and doesn’t participate in any physical activity. There are several great tools that will help you determine how many calories you need to consume, one of them is : http://www.calculator.net/calorie-calculator.html
You may heard of the terms calorie deficit and calorie surplus but what exactly are they? In order to lose weight you have to go on a calorie deficit that means less calories then your body needs and calorie surplus to put on weight. Your body takes up certain amount of calories just to function (BMR) and then at the top of that any physical activity that you do. How do you calculate the calories that your body needs to stay the same? Again there is plenty of free tools that will do that for example: https://www.freedieting.com/calorie-calculator or the one I mentioned above.
Widely used estimate is that for every 3500 calories deficit you will lose 1 pound of weight. It’s a rough estimate however the real number will depend on the individual so just experiment. If you’re not losing weight, it means you are still on a calorie surplus, reevaluate where you are making mistakes and cut them out.