A flash flood warning is an alert from the U.S. National Weather Service (NWS) that a flash flood is imminent or already happening in the warned area. It calls for immediate action: people in flood-prone places should move to higher ground right away. A flash flood is a sudden, violent flood that can develop within minutes to hours, and it can even strike places that are not getting rain at the moment.
A warning is more serious than a flood watch, which only means conditions are favorable for flooding, and a flood advisory, which covers flooding expected to be a nuisance rather than a threat. The NWS says it is moving flash flood warnings to an impact-based format to improve how the public responds.
Source: National Weather Service: Flood Warning vs. Watch
Image: Flash flood warning sign in Death Valley, by Draxfelton, public domain, via Wikimedia Commons.