Literally: lightning – light – stone – fire
Alternately: As quick as lightning or sparks struck from a stone. Doing something with incredible speed, or something happening in an extremely short span of time.
Notes: Apparently some people replace 石火 with a homophone such as 石下, but this is an error.
This compound is attributed to a 13th-century history of zen Buddhism in China known as the 五灯会元.