The plasma causing the ball lightning is thought to be sustained by the absorption of radio waves, which are generated during electrical storms. As electrons are recaptured by ionised atoms, they emit light, rendering the plasma luminous. Your correspondent asks if their lives were in danger. They certainly were. There is a report that a scientist studying one of Benjamin Franklin's early experiments was killed by ball lightning.
Ball lightning is a very peculiar and rare phenomenon.
It occasionally appears during or around the time of thunderstorms as a hovering, grapefruit-sized ball of light. In addition to the characteristics mentioned above, it is apparently capable of moving through windows and walls, and against airflow. The ball can continue glowing for several minutes before suddenly disappearing. The theory about radio waves, mentioned above by Robert Thurstands, is one of a category of theories in which an external source of energy maintains the glowing ball of light. Radio waves are one possible source, others are a flow of current from cloud to ground or even focused cosmic ray particles.