I think it's something to do with how fast/slow you're going, but some clarification would be nice.
Google Maps Navigation also provides Live Traffic Information along your route, updated every few minutes. A red/amber/green light in the corner shows if there are any traffic problems anticipated along your route. If there are, you can see where the holdups are and have the option to press to re-route which will find a route avoiding the traffic holdups.
(Via: Google Search For "google navigation amber green light")