It's an old word that comes from the Middle English word to denote a river or a stream which in itself derived from Latin and flumen, meaning “to flow”. Modern day usage refers to a water channel (for example, transporting logs down a river) or a water slide.
The Flume for Mac design aims to replicate this idea, with a thin channel (window) of photos and videos that slide down to the side of your desktop all-day, and the Flume logo also hints to this meaning with the blue tones and ripple effect background.
If you're not a native English speaker, you can listen to its pronunciation.