A calorie is a unit of energy. It is a unit of heat used to indicate the amount of energy that foods will produce in the human body.
Whether you lift weights/work out or not it is important to know how many calories to ingest a day, this way you are able to sustain the right amount of energy throughout the day.
If you feel happy about your current weight while having enough energy throughout the day, you are properly exercising weight-maintenance. In a clinical setting, weight maintenance might refer to maintenance of current weight, weight after weight loss, or some other selected target weight.