Literally: electricity – light – stone – fire
Alternately: A very short span of time. Alternately, incredible speed of motion. “Lightning-flash and sparks (struck from a stone),” as examples of things that happen quickly.
Notes: The parts can also be given in reverse order, 石火電光 (sekka-denkou). Apparently some people write sekka as 石下 (stone – down), but this loses the meaning of sparks and is an error.