Calories is just a random weird number that has something to do with how healthy food is for anyone who hasn't bothered to put the effort into finding out what it all means.
On average males burns 2,500 calories a day whilst females burn 2,000 calories. 2,500 calories is the same as 10 basic McDonald burgers. In comparison you would need to eat 4 kg of salad to reach 2,500 calories. Something more relatable would be how a banana is around 100 calories. I could continue on and on listing all sorts of food items and their calories, however that still wouldn't explain what calories represent.
Calories are used to measure the energy contained within food. All foods are listed in kilocalories so 1 of those calories is actually 1,000 normal calories. 1 (1000) calories is the amount of energy needed to increase the temperature of 1 kg (1 liter) of water by 1 degree Celsius. These calories are stored within the body as a kind of fuel that will be used up throughout a normal day. Lots of people try to decrease their calorie intake to lose weight which is pretty sensible. If you don't eat, you can't gain weight.
3,500 (3,500,000) excess calories result in one pound gained.
However you really shouldn't concentrate on the exact calorie amounts when you decide to eat something. You will be able to tell if your food choice is healthy or not regardless. If you're skinny, eat a bit more, if you want to lose weight, find out what a reasonable portion is. Labels are legally allowed to have a 20% margin of error. So that makes calculating pretty pointless. And these results are based on averages and estimates. They didn't explicitly check your banana to tell you how many calories it contains. A further area of imprecision is that up to 20% of the calories that we consume won't be absorbed. It's also just a big waste of time and may result in you avoiding some healthy food options.