Most simply, Go foods are any foods that give people energy. Carbohydrates like cereal, bread, pasta, rice, and potatoes are all good sources of energy that are often called Go foods. These foods give our bodies the energy to run and jump and work, as well as Vitamin B, which helps to convert that food into energy needed by our muscles.
All foods 'have' kilojoules. A kilojoule is a measure of the amount of energy they can release when we 'burn' them. (You may be more familiar with calories, which are a different sized unit for the same thing). More exactly, all foods release energy when we use them in respiration, and this energy can be measured in kilojoules.