Complex sentence: A gamma-ray burst is a highly luminous flash of gamma rays, the most energetic form of electromagnetic radiation, often followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).
Split into simpler sentences: A gamma-ray burst is a highly luminous flash of gamma rays, the most energetic form of electromagnetic radiation. This is often followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio).