Many people grew up hearing the term heat lightning during the summer time, the old tale of the heat and humidity during the summer months somehow make their own lightning. Well that is all just a myth. What you are actually seeing is lightning from a distant thunderstorm it is just too far away for you to see the cloud-to-ground flash or hear the sound of thunder.
Mountains, hills, trees, or just the curvature of the earth can prevent you from seeing the flash, instead the light is reflected off high-level clouds. This can travel up to 100 miles while the sound of thunder only travels 10 miles. When you see this at night time during the summer months if you check the radar there will be a thunderstorm somewhere, it might be 100 miles away and across state lines but you will still be able to see the lightning from it.