It is the static charge that builds up across the face of the monitor and is due to the high voltage applied to phosphors to make them glow. The static charge itself is not dangerous but voltages inside the monitor are extremely so. All monitors and TV sets that use a CRT do this. The only way to prevent it is to not use it. If you need to clean the monitor, turn it off and wait for at least 30 minutes for the charge to dissipate. Otherwise, You will just have to endure the small shock.