In simple words, calories are the measurement of energy contained in food or drinks. It is important to maintain a good balance between calories intake and with those burnt out.
On average an adult male needs about 2,500 calories/day while an adult female needs about 2,000 calories/day to maintain her weight.
The amount of energy/calories we need depends on our age, lifestyle, exercise routine, height and weight.
If your BMI (Body Mass Index is over 25, you need to lose weight. If your BMI is less than 18.5, you need to gain weight. You can calculate the daily amount of calories you need in order to maintain a healthy weight here.