Cherenkov radiation is a type of electromagnetic radiation emitted by fissile material, such as in HORNET mines, when transitioning from Slipspace to realspace. It is extremely powerful, reaching across light-minutes. This is what gives Slipspace entry and exit the unique blue coloration, and usually alerts in-system enemy ships to their arrival. Covenant ships apparently also give off Cherenkov radiation, though whether it is emitted by plutonium or some other material on their ships is unknown. Cherenkov radiation is a flash of light produced when charged particles move through an insulator at a speed greater than the speed of light and is more well known as the producer of the blue glow related with nuclear reactors.
It also seems that either a material or the fusion core in the MJOLNIR armor produces Cherenkov radiation, with steps made with the Recon Armor to reduce the radiation emmited.