It is a concentration of heat, light or radiation so intense that its energy forms an event horizon and becomes self-trapped.
In other words, if enough radiation is aimed into a region of space, the concentration of energy can warp spacetime so much that it creates a black hole.
John Archibald Wheeler's 1955 Physical Review paper entitled "Geons" refers to the kugelblitz phenomenon and explores the idea of creating such particles (or toy models of particles) from spacetime curvature.
[2] A study published in Physical Review Letters in 2024 argues that the formation of a kugelblitz is impossible due to dissipative quantum effects like vacuum polarization, which prevent sufficient energy buildup to create an event horizon.
The kugelblitz phenomenon has been considered a possible basis for interstellar engines (drives) for future black hole starships.