Humans burn energy just by breathing, let alone all the energy required to work, think, run, hug, lift, cook, gather food, etc. Humans are biological machines that turn hamburgers and beer into more humans. In order to do that properly, we need to keep powering ourselves by ingesting food that we can digest for calories.
IN = OUT To maintain total body mass/weight, eat the same amount of calories as you burn.
IN > OUT To gain weight, eat more energy than you burn.
IN < OUT To lose weight, eat less energy than you burn.