A spark is a small, glowing particle that's either thrown off of a fire or that starts a fire. Your dad might tell everyone to stand back from the bonfire so they don't get burned by a spark.
Sparks are burning fragments that pop up off a fire, but they're also bright flashes, like the sparks of light reflecting off your sunglasses or electrical sparks given off by an engine. Some small thing that promises to grow larger is yet another kind of spark — you might notice a spark of anger within yourself when you see someone being treated unfairly, for example. The word spark comes from the Old English root spearca, "glowing or fiery particle thrown off."