means A small fiery particle thrown off by burning or friction, or by extension the small flash of energy, feeling, or inspiration that sets something larger in motion.

from From Old English 'spearca,' a small glowing particle thrown from fire — a word with cousins across the old Germanic tongues. The literal ember came first; only later did English let it leap into the figurative, so a clever person might be a 'bright spark' and romance could 'spark' between strangers, the metaphor borrowed straight from the way one stray ember can catch and grow.