The phrase "cool summer nights" refers to warm, pleasant evenings during the summertime. It is a time when temperatures are not too hot and there is often a gentle breeze that makes it feel comfortable outside. These types of nights are perfect for spending time outdoors with friends or family, enjoying activities such as barbecues, bonfires, or simply relaxing in the fresh air. The phrase also evokes a sense of calm and tranquility, as the cool temperatures can bring about a peaceful atmosphere that is soothing to the senses.